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1
Make the cake: preheat the oven to 350u00b0F. Grease a 13x18-inch baking sheet with nonstick spray, line with parchment paper, then grease the parchment paper.
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2
In a medium bowl, whisk together the cocoa powder and boiling water. Add the milk and stir to combine.
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3
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar until light and fluffy, 4-5 minutes. Add the eggs one at a time, then beat in the egg yolk and vanilla.
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4
In a medium bowl, whisk the flour, baking powder, baking soda, and salt to combine. Add 1/3 of the flour mixture to the mixer and mix to combine. Add half of the cocoa mixture and mix to combine. Continue adding the flour mixture and cocoa mixture alternating until both are incorporated.
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5
Pour the batter into the prepared pan and spread into an even layer. Bake until the cake springs back lightly when touched in the very center, and a toothpick inserted into the center comes out clean, 20-23 minutes. Cool completely.
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6
Use a 2 inch round cutter to cut 27 rounds from the cake. Set aside.
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7
Make the filling: place the dark chocolate in a medium, heat-safe bowl. Bring the cream to a boil in a medium pot over medium heat, then pour over the chocolate. Let sit, undisturbed for 30 seconds, then mix until smooth. Let cool to room temperature.
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8
Whip the ganache in the bowl of an electric mixer fitted with the whip attachment until light and fluffy, 3-4 minutes.
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9
To build the cakes, place about 1 tablespoon of filling on each layer, and stack the cakes three layers tall. Use any remaining ganache to apply a light crumb coat to the sides of the cakes (this is optional, but will make the glaze look smoother on the finished cakes). Transfer the cakes to a wire rack over a baking sheet.
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10
Make the glaze: place the white chocolate in a medium, heat-safe bowl. Bring the cream to a boil in a medium pot over medium heat, then pour over the chocolate. Let sit, undisturbed for 30 seconds, then mix until smooth. Let cool for 15-20 minutes.
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11
Use a ladle to pour glaze over each cake. If desired, you can collect the remaining glaze from the bottom of the pan and apply a second coat for a thicker, smoother finish.
