Prepare the boiling water, then mix the cocoa with the water in a small bowl. Stir the mixture until smooth, then set aside to cool.
In the bowl of the stand mixer, beat the butter and sugar in until fluffy. Add each egg one at a time and beat well after each egg. Stir in the vanilla, then add the cocoa mixture, stirring well.
In the medium bowl, stir together flour, baking soda and salt. In a liquid measuring cup, stir together the milk and sour cream. Pour about ⅓ the dry ingredients in the mixing bowl, then about ⅓ of the milk and sour cream mixture. Beat well. Continue until all of the ingredients are incorporated. Pour the batter into the prepared pan.
Bake 45 to 50 minutes or until wooden pick inserted in center comes out clean. Cool in the pan for 10 minutes, then remove the cake from the pan and cool completely on a wire rack. Dust with powdered sugar and serve.