In a large bowl stir together first six ingredients (through salt). Add milk, oil, and vanilla. Beat with a mixer on low just until combined. Beat on medium 2 minutes. Add egg; beat 2 minutes more. Spread batter in prepared pan.
Bake 30 to 35 minutes or until a toothpick comes out clean. Cool in pan on a wire rack 10 minutes. Remove cake from pan. While still warm, spread top with marshmallow creme and sprinkle with cracker crumbs or dust with additional cocoa powder. Cool on wire rack.