Heat oven 350℉/177℃.
Line 2 sheet pans with parchment paper. Set aside.
In a medium bowl, whisk together flour, cocoa powder, powdered milk, baking soda, and salt. Set aside.
In a small bowl, combine egg, milk, and vanilla. Set aside.
With an electric mixer, beat butter and sugar until light and fluffy.
Add egg mixture.
Beat until combined.
Stir in flour mixture.
Use a 2-inch cookie scoop to portion onto prepared pans, placing cookies 2-3 inches apart.
Flatten slightly.
Bake for 8 minutes, or until set.