• Because the pudding takes a long time to set fully, make the pudding first. It will need at minimum 4-6 hours in the refrigerator, so either start that early, or make it the day before you plan on serving the cake.

For the Pudding:

• In a large saucepan, pour in 1 cup whole milk, 2 cups heavy cream, ½ teaspoon salt, ¼ cup cornstarch, 1 cup granulated sugar, and ¼ cup brown sugar. Set over medium heat and whisk until the sugar has dissolved. Break or chop the 4oz unsweetened baking chocolate into pieces and add to the saucepan. Whisk constantly until the chocolate has fully melted and the mixture begins to bubble, about 3-5 minutes. You should feel it starting to thicken.
• Remove from the heat and whisk in 1 teaspoon vanilla extract. Pour into a non-reactive bowl and allow to cool to room temperature. Cover with plastic wrap, placing the wrap directly onto the surface of the pudding, and refrigerate until set, at least 4-6 hours or overnight.

For the Cake:

• Place oven rack in the middle position and preheat your oven to 325°F / 165°C. Grease two 8-inch round cake pans, add a parchment round and grease that, and then dust with cocoa powder. Alternatively, use a floured baking spray.
• In a mixing bowl, add 1 ½ cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon salt. Whisk to combine and set aside.
• Melt ½ cup unsalted butter in a heat-proof container like a liquid measuring cup and add ¾ cup Dutch-processed cocoa powder while still hot. Mix until the mixture turns fudgy and pour into a large mixing bowl. Pour in ¾ cup whole milk, 1 cup white and 1 cup brown sugars, and ¾ cup sour cream and whisk until everything is combined. Whisk in the eggs, one at a time, followed by 1 teaspoon vanilla until the mixture is smooth and combined.
• Add the dry ingredients into the wet mixture, about ⅓ at a time, until fully combined. Pour the batter evenly into the prepared cake pans and then bake for 30-35 minutes. A toothpick inserted should come out mostly clean when they’re done. Remove from the oven and let cool in the cake pans at least 5 minutes, then move to a cooling rack. Allow to cool completely before frosting, at least 1-2 hours.

For Assembly:

• Once cooled, use a large, serrated knife or cake leveler to slice both cakes in half horizontally. Take one cake layer and break into small crumbles, either by hand or in a food processor. On a cake stand or turntable, add some strips of parchment or wax paper and place one cake layer on top. Add 1 cup of frosting on top and smooth it out with an offset spatula, bringing it all the way to the edge and leveling it out. Place the next cake layer on top and press down lightly. Add another cup of frosting and repeat. Place the final cake layer on top and cover the tops and sides evenly with the remaining frosting. Sprinkle the cake crumbs over the top and sides and press lightly into the frosting so they stick.
• Refrigerate at least 20 minutes before serving. Leftovers can be refrigerated for 2-3 days.