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To make the pumpkin brown butter cake, start by preheating the oven to 325 degrees F. Grease the bottom and sides of two 8-inch round cake pans. Place a round piece of parchment paper on the bottom of each cake pan and then grease the top of the parchment paper. Flour the sides and bottom of the cake pans, set aside until needed.
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In a small saucepan, melt the butter over medium-low heat. Continue to cook until the butter has turned a golden brown color. There will be small black dots in the butter. Don't worry, this normally happens when you are browning butter.
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Place a mesh strainer over a small bowl and pour the browned butter through the strainer. This will catch most of the burnt pieces in the butter. Set the bowl aside to cool.
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In a medium bowl, whisk together the flour, baking soda, cinnamon, ginger, allspice, and salt. Set aside.
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In the bowl of a stand mixer, beat together the sugar, brown sugar, pumpkin puree, eggs, and sour cream. With the mixer running on low speed, slowly add the flour mixture. Add the brown butter and mix until it is fully incorporated and the batter is smooth.
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Divide the batter between the prepared cake pans. I use a scale to weigh the pans and make sure they are even. Smooth the tops of the batter and place the pans in the oven to bake. Bake until the tops turn a light golden brown and a toothpick inserted into the middle of each cake comes out clean, about 30-40 minutes.
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Remove the cakes from the oven and let sit in the pans for 10 minutes. Carefully remove the cakes from the pans and remove the parchment paper from the bottom of each cake. Place the cakes on a cooling rack to cool completely.
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While the cakes are cooling, make the toasted marshmallow filling. Turn the oven temperature up to broil, or 500 degrees F if you don't have a broiler. Place the marshmallows on a baking sheet lined with parchment paper or a silicone mat.
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Broil the marshmallows until the tops turn a light golden brown. Remove the baking sheet from the oven and carefully turn the marshmallows over. Place the marshmallows back in the broiler and bake until they turn golden brown. Remove the marshmallows from the oven and let sit until cool (they will deflate while cooling).
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In the bowl of a stand mixer, beat together the butter and confectioner's sugar until light and fluffy. Add the vanilla and mix until incorporated. Add the toasted marshmallows and marshmallow fluff and mix on medium-low speed until the filling is smooth and fluffy.
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Set the filling aside, and then make the cinnamon Swiss meringue buttercream. In the bowl of a stand mixer, combine the egg whites, sugar, and salt. Place the bowl over a pot of simmering water. Whisk constantly (I use a hand mixer) until the sugar is dissolved and the mixture reaches about 150 degrees F.
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Take the bowl and attach it your mixer, fitted with the whisk attachment. Whisk the egg white mixture until it reaches room temperature and stiff peaks have formed, about 5-10 minutes. At this point the mixture will be thickened and white.
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With the mixer on low speed, add the butter, 2 tablespoon as a time. Once all the butter is added, raise the speed to medium high and mix until the mixture starts to come together. Remove the whisk attachment and replace with the beater attachment. Beat until the buttercream comes together fully and is smooth. This can take time, sometimes 2 minutes, and sometimes up to 10 minutes. Don't worry if you think you did something wrong, it will come together and create a smooth and creamy buttercream.
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Once the buttercream is silky and smooth, mix in the vanilla. Then add the cinnamon and mix until it has spread throughout the whole buttercream.
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Before assembling the cake, wrap each cake in plastic wrap and place in the freezer for about 20 minutes, just to firm it up. Cut each cake horizontally in half using a large serrated knife. This will create 4 even layers.
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To assemble the cake, place 1 cake on a cake stand or plate, bottom side down. (I assembled my cake on a cake turntable with a small cardboard on the bottom of the cake.) Spread 1/3 of the marshmallow filling on top of the cake. Place another cake on top, bottom/flat side up and spread 1/3 of the filling on top. Repeat with the third cake.
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Top with the last cake and using the cinnamon buttercream, frost a thin layer around the sides and top of the cake to seal in the crumbs (this is a crumb coat). Place the cake in the refrigerator, or freezer, to harden the crumb coat. Finish by frosting the cake with an even layer of buttercream on the sides and top of the cake, leaving about 1 cup of buttercream for decoration. Evenly smooth the buttercream with an offset spatula.
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To create the decoration on top, place the leftover buttercream in a piping bag fitted with a medium petal tip. Starting at one point on the top of the cake, hold the piping bag with the larger part of the tip facing in towards the center of the cake. Lightly applying pressure, move your piping bag in a back and forth motion, about 2-3 times upwards. Move onto the next row and do the same motion until you finish ruffling around the sides of the top of the cake.
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To create the dots on the bottom of the cake, change the tip of the piping bag to a small round piping top. Pipe small dots around the bottom of the cake.
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Place the cake back into the refrigerator or freezer to slightly firm up. Slice, serve, and enjoy!
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Recipe sources: Pumpkin Brown Butter Cake from Sprinkle Bakes; Toasted Marshmallow Filling from Sweetapolita; Swiss Meringue Buttercream adapted from Martha Stewart.
