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1
Preheat the oven to 350u00b0F. Lightly grease two 8 inch cake pans with nonstick spray, then add cocoa powder and tap around until the pan is well coated.
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2
Place the 1 1/2 cups (128 g) cocoa powder in a medium, heat-proof bowl. Whisk in the boiling water until fully combined. Stir in the milk and set aside.
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3
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar on medium-low speed until light and fluffy, 4-5 minutes. Add the eggs one at a time, mixing until each is fully incorporated before adding the next. Scrape the bowl well. Add the vanilla and mix to combine.
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4
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Add one third of the mixture to the mixer and mix on low speed to combine. Add half of the cocoa/milk mixture and mix to incorporate. Repeat, alternating between the dry and wet ingredients, until both are fully incorporated and the batter is smooth.
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5
Divide the batter between the prepared pans. Bake the cakes until a toothpick inserted into the center comes out clean, 35-40 minutes. Cool the cakes in the pan for 10-15 minutes, then turn out onto a wire rack to cool completely.
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6
While the cakes cool, make the filling: in a medium pot, heat the milk, cream, and chocolate over medium-low heat, stirring constantly, until the chocolate has fully melted. Turn the heat up to medium and bring the mixture until it's just barely simmering.
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7
Meanwhile, whisk the sugar, cornstarch, and salt together in a medium heatproof bowl. Whisk in the egg yolks until well combined.
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8
When the chocolate mixture has come to a bare simmer, pour about half of it into the egg yolk mixture in a slow, steady stream, whisking constantly. Then pour the egg yolk mixture into the saucepan and cook the pudding over low heat, stirring constantly with a spatula until it gets very thick and a few large bubbles break the surface. Stir in the butter and vanilla until well combined.
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9
Pour the mixture onto a sheet tray, or into a shallow bowl or casserole dish. Cover directly with plastic wrap and refrigerate until totally chilled.
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10
Make the whipped ganache: place the chocolate in a medium heatproof bowl. Bring the cream to a boil in a small pot, then pour it over the chocolate. Stir until the mixture is smooth, then let it cool to room temperature (don't let it set until it is totally firm).
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11
Transfer the ganache to the bowl of a stand mixer fitted with the whisk attachment, and whip on medium high speed until lightened in color, 2-3 minutes. Scrape the bowl well and whip again to make sure the ganache is fully aerated. Transfer half of the ganache to a disposable piping bag (or a large zip-top bag). Cut a 1/2 inch opening in the end, or fit with a large round tip.
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12
Assemble the cake: use a serrated knife to divide each cake evenly into two pieces. Place one half on a cake turntable, cake stand, or platter. Pipe a ring of icing around the outer edge of the cake layer. Scoop 1/3 of the pudding mixture inside (don't let it rise over the ganache ring around the outside), and spread into an even layer. Place another cake layer on top. Repeat this process until you've used all of the pudding and cake layers.
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13
Use the remaining whipped ganache and/or custard to do a thin crumb coat all around the outside of the cake. Chill the assembled cake for 15-30 minutes.
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14
Spoon or pour the glaze all over the top, letting it drip down the sides. After the glaze sets, you can pipe the remaining whipped ganache in dots (or switch to a star tip - whatever!) on the top of the cake, then finish with sprinkles.
