Preheat the oven to 350 degrees F. Spray two 9x2 -inch metal cake pans with flour-added baking spray or grease with butter/shortening and flour.
Mix together the flour, baking powder and salt and set aside.
In a stand mixing bowl, beat the butter on medium high speed until light and creamy. Beat in the vanilla. Gradually add the sugar, scraping the sides of the bowl once or twice and beating for about 5 minutes or until light and creamy. Add eggs one at a time and continue beating for another minute or two, scraping sides of bowl. Reduce speed to low and add the milk, then add flour and sour cream alternately. When flour is fully incorporated, divide evenly among the pans.
Bake on center rack for about 35 minutes or until cakes spring back when touched and a pick inserted comes out with moist crumbs.
To make the frosting, mix together the butter, cocoa powder and 2 1/2 cups confectioners' sugar until the come together. Add a little salt. Bring mixer speed from low to high, scraping sides of bowl, until well mixed, then gradually add the sour cream. Add milk. Taste, and add the remaining 1/2 cup confectioners sugar if you feel you need it.