Preheat the oven to 350 degrees F. Line muffin tins with paper liners.
Divide the egg yolks from the white and put them in separate bowls. Beat the egg yolks and the sour cream together. Gradually beat in the sugar and beat until it thickens.
Combine the hot water, cocoa, and butter together heat over low heat until the butter melts. Remove from heat and cool. Add to the egg mixture and mix in. Mix in the vanilla.
Sift together the cake flour, baking soda, and salt. Add the wet ingredients and mix in.
With clean beaters beat the egg whites until stiff. Fold into the cake batter.
Fill the paper liners to 3/4 full. Place the cupcakes in the preheated oven.
Bake for about 20 minutes or until a wooden pick inserted comes out clean. Remove and cool on a cooling rack.