Step 1: Prepare the Cake Layers

To start, preheat your oven to 350°F (175°C). Grease and line two 8-inch cake pans with parchment paper. In a large mixing bowl, combine flour, cocoa powder, baking soda, and salt. Mix thoroughly.

In another bowl, whisk together granulated sugar, brown sugar, eggs, buttermilk, oil, and vanilla extract until smooth. Gradually add the dry ingredients into the wet ingredients, mixing until fully incorporated.

Once the batter is smooth, stir in the boiling water (it will be thin, which is perfect for a moist cake). Pour the batter evenly into your prepared cake pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes before transferring them to a cooling rack to cool completely.

Step 2: Make the Chocolate Pudding Filling

In a medium saucepan, combine the milk, cornstarch, sugar, and salt. Whisk together until smooth. Heat the mixture over medium heat, constantly stirring, until it begins to thicken.

In a separate bowl, whisk the egg yolks. Once the milk mixture thickens, slowly pour some of it into the egg yolks to temper them. Then, return the egg mixture to the saucepan and cook for an additional 2-3 minutes until thick and smooth.

Remove from heat and stir in the butter, chopped chocolate, and vanilla extract. Allow the pudding to cool to room temperature before using it in the cake.

Step 3: Make the Chocolate Frosting

In a stand mixer, beat the butter until smooth and creamy. Gradually add the powdered sugar, cocoa powder, and milk, mixing until smooth. Add the melted chocolate and vanilla extract, then beat until the frosting is fluffy and spreadable. If it’s too thick, add more milk, one tablespoon at a time, until it reaches the desired consistency.

Step 4: Assemble the Cake

Once the cake layers are fully cooled, place one layer on a cake plate. Spread a generous amount of chocolate pudding filling over the first layer. Top it with the second layer of cake. Then, frost the top and sides of the cake with the chocolate frosting, making sure to cover it entirely.

Step 5: Decorate and Serve

For a professional touch, pipe additional frosting around the edges and top of the cake. You can also sprinkle extra chocolate shavings or cocoa powder over the cake for an added layer of flavor. Let the cake sit for about 30 minutes to allow the frosting to set before slicing.