Bake at 350° for 45-50 minutes, or till a toothpick comes out clean.
Let cool for 10 minutes in the pan, then invert to a cooling rack. Cool completely.
Frosting:
Combine butter, cocoa, salt, powdered sugar, vanilla, and 3 Tbsp milk. Beat till smooth, adding enough milk to get a spreadable consistency. It should be thin enough that it will run down the sides of the cake a little, but not be as thin as a glaze.