Brooklyn Blackout Cake

For the Salted Dark Chocolate Pudding
¼ cup (57g) plus 1¾ cups (398g) whole milk
2½ Tbsp. cornstarch
½ cup (99g) sugar
2 oz. (56g; about ⅓ cup) dark chocolate, chopped
3 Tbsp. dark Dutch-process cocoa powder
1 tsp. vanilla extract
¾ tsp. sea salt
For the Black Chocolate Stout Cake
1½ cups (340g) Brooklyn Brewery Black Chocolate Stout, or other stout of choice
1½ cups (340g) unsalted butter, cut into ½-inch pieces
1½ cups (126g) dark Dutch-process cocoa powder
3 cups (360g) all-purpose flour
2¾ cups (544g) sugar
¾ Tbsp. baking soda
1½ tsp. salt
1 cup (227g) sour cream
3 large eggs, at room temperature
For the Dark Chocolate Pudding Buttercream
16 Tbsp. (1 cup, 8 oz.) cold unsalted butter
7 cups (795g) confectioners’ sugar, plus more for thickening
¾ cup Salted Dark Chocolate Pudding
½ cup (42g) dark Dutch-process cocoa powder
¼ tsp. salt

Make the Salted Dark Chocolate Pudding
In a small bowl, whisk together the ¼ cup whole milk and the cornstarch until smooth. Set aside.
In a medium saucepan, combine the remaining 1¾ cups milk, sugar, dark chocolate, dark Dutch-process cocoa powder, vanilla extract and sea salt. Heat over medium-low heat, whisking, until the chocolate is melted.
Whisk the cornstarch mixture into the chocolate mixture until fully incorporated.
Reduce the heat to low, and continue to stir briskly with a wooden spoon or a heatproof spatula. The mixture will come to a simmer and will slowly begin to thicken.
Continue to cook for 1 to 2 minutes, or until the pudding coats the back of the spoon and slowly drips off. It will be thick and will just be starting to bubble. Remove the pudding from the heat and pour into 4 serving ramekins or bowls. Let cool.
Cover with plastic wrap, refrigerate the pudding until it sets, and serve.
Make the Dark Chocolate Pudding Buttercream
Cut the cold butter into ½-inch pieces. Let it come to room temperature.
In the bowl of a stand mixer fitted with a paddle attachment, combine the butter, 3 cups of the confectioners’ sugar, ½ cup dark Dutch-process cocoa powder, ¼ teaspoon salt, and ¾ cup Salted Dark Chocolate Pudding, and mix on low until just incorporated. Then beat on medium-high until the mixture is creamy and ingredients are incorporated, about 1 minute. Scrape down the sides of the bowl with a rubber spatula.
Add more confectioners’ sugar, 1 cup at a time, and mix on low until the frosting is thick but spreadable. Beat for 1 minute after each addition. You may not need to add all the remaining sugar. Once you have your desired consistency, scrape down the sides of the bowl.
Raise the speed to medium-high, and beat for 3 to 4 minutes, or until very light and fluffy. The buttercream should be thick but spreadable. If the buttercream appears too thick, add more cream, 1 tablespoon at a time. If it appears too thin, add more confectioners’ sugar, 1 heaping tablespoon at a time.
Make the Black Chocolate Stout Cake
Preheat the oven to 350ºF. Grease two 9-inch cake pans with butter and dust with flour. Line with parchment rounds and grease the rounds.
In a large heavy saucepan over medium heat, bring the stout and unsalted butter to a simmer. (You can also melt the butter in your oven or in a large, microwave-safe bowl in a microwave oven and then whisk in the beer.) Remove the stout-butter mixture from the heat, add the Dutch-process cocoa powder and whisk until the mixture is smooth. Let cool for 5 minutes.
While the stout-butter mixture cools, in a large bowl, whisk together the flour, sugar, baking soda and salt.
In a separate large bowl, whisk together the sour cream and eggs.
Add the stout-butter mixture to the egg mixture and whisk to combine. Then add the flour mixture, and combine with a rubber spatula until all the ingredients are incorporated and the batter is smooth, with no lumps. Be sure to scrape the bottom of the bowl to incorporate any dry flour bits.
Divide the batter equally between the prepared cake pans. Bake for 35 to 40 minutes, or until a toothpick inserted in the center of each layer comes out clean. Transfer to a rack to cool.
Finish the Brooklyn Blackout Cake
Frost the cake with the Dark Chocolate Pudding Buttercream.