Preheat your oven to 350°F (175°C). Grease two 9-inch cake pans with butter. Line the bottoms with parchment paper. This helps the cake come out easily. Dust the sides with a little cocoa powder.

In a large bowl, whisk the sugar and flour together. Add the cocoa powder, baking powder, and baking soda. Add the salt and stir well. Make sure there are no large lumps in the mixture.

Add the eggs, milk, oil, and vanilla extract to the dry bowl. Use an electric mixer on medium speed. Beat the mixture for about two minutes. The batter will look thick and smooth at this stage.

Carefully pour in the one cup of boiling water. Stir it in by hand using a whisk. The batter will become very thin and watery. This is normal for this recipe. It ensures a moist texture.

Pour the batter evenly into the two prepared pans. Place them in the center of the oven. Bake for 30 to 35 minutes. A toothpick should come out clean when poked in the middle.

Let the cakes cool in the pans for ten minutes. Then, turn them out onto a wire rack. Remove the parchment paper carefully. Let them cool completely before you start frosting them.

Whisk the melted butter and cocoa powder together. Alternately add powdered sugar and milk. Beat on medium speed until the frosting is spreadable. Add vanilla and a splash more milk if needed.

Place one cake layer on a plate. Spread a thick layer of frosting on top. Place the second layer on top of that. Cover the top and sides with the remaining chocolate frosting.