Preheat your oven to 350°F (175°C). Coat two 9-inch cake pans with parchment paper so that they can easily be removed.

Combine butter and sugar in a large mixing bowl and cream together to a light-fluffy mixture. This is done to add air to the batter to provide the cake's structure.

Add the eggs sequentially with proper mixing and add vanilla extract.

In a small saucepan, melt the semi-sweet chocolate over low heat. Allow to cool a little, then pour it into the batter (or it will scramble the eggs!).

Sift the flour, cocoa powder, baking powder, baking soda, and salt in another bowl.

Combine the dry ingredients into the butter mixture; each time adding sour cream followed by the dry. Stir until just combined.

Split up the batter into the cake pans. Bake 30-35 minutes, or until a toothpick in the center of the cake draws out, not drippy batter, but moist crumbs.

Allow cakes to rest in pans 10 minutes before removing them onto wire racks to cool.

In a saucepan, melt the butter and the chocolate until smooth. Take off the heat and add powdered sugar, milk, vanilla, and salt, and blend until it is creamy and spreadable.

Frost between the layers, and on the sides and top. Apply with an offset spatula to produce that glossy, smooth appearance.