Mix flour, cocoa powder, baking soda, and salt together in one bowl.
In another bowl cream butter and sugar.
Add sour cream, egg, water, and vanilla. Mix well.
Add flour mixture to butter and sugar mixture and mix well.
Fill lined cupcake holders 1/2 full.
Bake for 20-25 minutes or until a toothpick comes out clean after inserting into the center of a cupcake.
Set on a rack to cool.
Meanwhile make the chocolate frosting by mixing butter, cocoa powder, and powdered sugar together. Add milk and mix well. Add more milk if the frosting seems too thick.
Nutritional information is provided as a courtesy and is an estimate only. This information comes from online calculators. Although attempts have been made to provide accurate nutritional information, these figures are only estimates.