Preheat the oven to 350 F.
Lightly grease, or oil a 9x12 baking dish, then set to the side
In a large mixing bowl, cream the butter, and sugar together until it’s light and fluffy.
Add each egg, one at a time, and beat well after each addition.
Stir in vanilla extract.
Sift the flour, cocoa powder, baking soda, and salt into another large mixing bowl. Whisk to combine.
Add the wet mixture and sour cream to the dry ingredients and beat with an electric mixer just until well blended.
Add the boiling water then stir until combined.
Pour the batter evenly into the prepared baking dish.
Bake on 350 F for 45 minutes, or until a toothpick is inserted into the center of the cake and comes out clean.
Set the cake aside to cool.
Melt the butter, then stir in the semi-sweet and unsweetened chocolate.
Microwave in 30 second intervals, stirring between each interval until the chocolate is melted.
In a large mixing bowl, combine the melted chocolate and butter mixture, powdered sugar, sour cream, and vanilla.
Beat with an electric mixer until smooth.
Spread the frosting evenly onto the cooled to room temperature cake.