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For the cowboy sourdough cake:
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In a large mixing bowl, combine fed sourdough starter, buttermilk, and all-purpose flour. Cover the bowl loosely with plastic wrap and let the mixture sit at room temperature until it has expanded a little bit, about 2 to 3 hours.
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Preheat oven to 350u00b0F and grease two 8-inch cake pans with baking spray, set aside.
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In the bowl of a stand mixer fitted with a paddle attachment, beat the sugar and butter at a medium speed until pale and fluffy, about 2 to 4 minutes. Scrape the sides and bottom of the bowl, add the oil and mixture until blended. On a low speed mix in the Parmesan, baking soda, salt, and vanilla. The mixture will look a bit granular. Add the eggs, one at a time, beating well after each addition.
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Using a rubber spatula, slowly and gently add the wet mixture in the starter-flour-milk mixture. Stir until smooth.
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Pour the batter into the prepared pans and bake for 25-30 minutes until a toothpick in the center comes out clean with a few moist crumbs. Let the cakes cool in the pans for 5 minutes before turning them onto a wire rack to cool completely.
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For the cookie butter Swiss meringue buttercream:
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Put the egg whites in a stand mixer's bowl laying above water simmering in a small sauce pot, creating a double boiler. The bowl should not have contact with the water below. Lightly whisk the egg whites and sugar until a thermometer reads 160u00b0F. Fit the bowl to the stand mixer.
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Using the whisk attachment, whip the egg white mixture on a medium-high speed until stiff peaks form, about 8-10 minutes. At this point the mixer's bowl should have cooled down to room temperature. Switch the whisk to the paddle attachment.
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Turn the mixer back on, this time to a low speed. Add butter, a few tablespoons at a time, followed by the vanilla. Once added, raise the speed to a medium-high speed and beat until silky, smooth, and lump-free, about 3-5 minutes. Add the cookie butter and beat until incorporated.
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For the blue lasso frosting:
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Put the cream cheese and butter in a stand mixer fitted with a paddle attachment. Beat on a medium-high speed until pale in color and lump-free. Add vanilla and then scrape the sides and bottom of the bowl.
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With the mixer's speed on low, slowly add powdered sugar, scraping the bowl as needed. Beat again on a medium-high speed for a final 1 to 2 minutes. Add blue gel until the desired shade is reached.
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To assemble the cowboy sourdough cake:
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If the cake layers have domes, use a serrated knife to level off the tops. Put a dollop of frosting on the cake board. Place one layer on, spread on the blue lasso frosting, and then put on the second cake layer. Use an offset spatula to cover the cake with a dirty layer of the Swiss meringue buttercream. Place the cake in the freezer for 15 minutes. Apply a second thicker layer of the Swiss meringue buttercream. Chill again for another 15 minutes.
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Put blue lasso frosting in a pastry bag fitted with a shell tip. Pipe borders on the top and bottom of the cake.
