In a medium bowl, mix together flour, cocoa powder, baking soda, and salt with a whisk. Set aside.
Using an electric mixer, beat butter on medium-high setting using a paddle attachment until creamy. Gradually add sugar and beat on high until well blended. Reduce speed to medium, add eggs, one at a time, and then vanilla, until blended.
Reduce speed to low, then mix in half of dry ingredients slowly, then sour cream, and then the remaining dry ingredients until well blended.
Scoop batter into lined baking cups, filling each about 2/3 full. Bake for 20 minutes, or until a toothpick inserted into center of a cupcake comes out cleanly. Cool cupcakes in pan for 15 minutes, then remove to a wire rack to cool completely.
Using an electric mixer, mix butter and cocoa powder together on low speed. Add powdered sugar very slowly, alternate blending in vanilla and chocolate milk. Mix well. If frosting is too thick, add in a teaspoon of chocolate milk to reach desired consistency.
Spread frosting on cupcakes, smoothing edges and top.
Pour green colored sugar into a shallow bowl. Invert frosted cupcakes, rolling each top into sugar to coat well.
Top with a football decoration and enjoy!