Mix classic cookie dough with 1/4 cup of cocoa along with the flour. Follow classic cookie dough recipe, cutting into 1x2" bars.
Melt 1/4 cup dark chocolate chips in a small microwaveable bowl on high for 1 minute., then whisk until smooth. Let cool to room temperature. Using mixer, beat together 1 1/2 cups of icing sugar, 2 tbsp room temperature unsalted butter, 3 tbsp milk, 1 tsp vanilla extract and 1/2 tsp mint extract until smooth and creamy, about 2 min. Add chocolate and beat until fluffy, about 2 min. Spread or pipe onto cookies and let set before serving.