For the Cake:

1. Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

2. In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.

3. In a large mixing bowl, cream together butter and sugar until light and fluffy, about 3-4 minutes.

4. Add eggs one at a time, beating well after each addition. Stir in vanilla extract.

5. Gradually add dry ingredients in three parts, alternating with milk. Mix until just combined.

6. Slowly add boiling water to the batter, mixing until smooth.

7. Pour batter evenly into cake pans and bake for 30-35 minutes, or until a toothpick inserted comes out clean.

8. Cool cakes in pans for 10 minutes, then transfer to a wire rack to cool completely.

For the Pudding Filling:

1. In a medium saucepan, whisk together sugar, cornstarch, salt, and cocoa powder.

2. Gradually whisk in milk. Cook over medium heat, whisking constantly, until it thickens (5-7 minutes).

3. Stir in egg yolks, then return to heat and cook for 2-3 minutes.

4. Remove from heat and stir in butter and vanilla. Cover with plastic wrap to prevent a skin from forming and let it cool to room temperature.

For the Frosting:

1. Beat butter until creamy. Gradually add powdered sugar and cocoa powder, mixing on low speed.

2. Add vanilla, salt, and heavy cream, and beat on medium-high speed until smooth and fluffy (about 3-5 minutes).

Assembling the Cake:

1. Slice each cooled cake layer in half horizontally to create four layers.

2. Place the first layer on a serving platter and spread chocolate pudding over it.

3. Continue layering the cake and pudding, stacking layers until all are used.

4. Frost the top and sides with the chocolate frosting. Optionally, decorate with crumbled cake or chocolate shavings