1 1/2cups340g unsalted butter, at room temperature
5 1/2cups690g confectioners’ sugar, sifted
1/2cup50g unsweetened cocoa powder
1 1/2teaspoonspure vanilla extract
1/4cup60ml heavy cream or whole milk
8ounces225 semisweet chocolate, melted and cooled
Make the butter cake: Preheat the oven to 350°F. Grease and flour two 8-inch cake pans and set aside.
Sift together the flour, baking powder and salt and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth. Add the sugar and mix on medium-high until the butter is light and fluffy, 3 to 5 minutes. Stop the mixer and scrape down the bowl.
Turn the mixer to medium-low and add the vanilla and egg yolks, one at a time. Stop the mixer and scrape down the bowl.
Turn the mixer to low and add the flour mixture in three batches, alternating with the milk, beginning and ending with the flour mixture. Mix on medium for no more than 30 seconds after the last streaks of the dry ingredients are combined.
Evenly divide the batter between the prepared pans. Bake for 25-28 minutes, or until a toothpick inserted into the center of the cakes comes out clean. Let them cool on a wire rack for 10 to 15 minutes before removing the cakes from their pans.
Make the fudge frosting: In the bowl of a stand mixer fitted with the paddle attachment, beat the butter until smooth and creamy. With the mixer on low, gradually add the confectioners’ sugar, cocoa powder, vanilla and salt. Pour in the cream and mix until incorporated. Turn the mixer to high and mix until the frosting is light and fluffy. Stop the mixer and scrape down the bowl. Add the chocolate and mix until smooth.
Assemble the cake: Once the cakes have completely cooled, carefully halve them horizontally to create four even layers of cake (see Baker’s Notes following). Level the cakes and choose which layer will be at the bottom. Place it on a plate or serving dish. Spread on 3/4 cup (180 ml) of the fudge frosting with an offset spatula. Place the next layer of cake on top and repeat. Frost with the remaining fudge frosting.
Decorate it: Toss on some colorful sprinkles or add a handful of candles for an extra-festive element.
If you would prefer a two-layer cake, keep the two cakes whole. Adjust the filling amount accordingly, using 1 to 1 1/2 cups (240 to 360 ml) of the fudge frosting between the two layers. The cake will keep in the fridge for up to 4 days; it may also be frozen.