Step 1

For the cake:Preheat the oven to 350 degrees and lightly grease a 9 inch cake pan that's at least 2 inches deep.Place the sugar and butter into a large bowl. Beat the two together until they are thoroughly combined. Add one egg to the bowl with the butter and sugar, beat until well combined. Add the second egg to the bowl.  After you've added the second egg, beat it in with your electric mixer on high speed for 2 minutes; the batter will lighten in color and become fluffy. Add the vanilla extract, salt, baking powder, and baking soda, stirring to combine.Alternately add the flour and sour cream to the mixture. Add 1/3 of the flour, beating until it's just combine, add half the sour cream, beating until just combined, add another  1/3 of the flour, beating until just combine, the remaining sour cream, again beating until just combined and the remaining flour, beating until just combined.Spoon the batter into the pan. Bake the cake for 30 to 35 minutes, until it's golden brown on top, the edges are beginning to pull away from the sides of the pan, and a toothpick inserted into the center comes out clean. Remove the cake from the oven to a wire rack to cool for 10 minutes. After 10 minutes, turn the cake out of the pan onto a rack to cool completely before frosting.

Step 2

Melt the butter in a small saucepan over low heat. Add the cocoa, cook, stirring constantly, just until the mixture begins to boil. Pour the mixture into a medium bowl, cool completely. Add powdered sugar alternately with milk to cocoa mixture, beating to a spreading consistency. Stir in vanilla.