Preheat oven to 350°F. In a microwave-safe bowl, heat butter, 15 seconds at a time, until softened but not fully melted. In a large mixing bowl, combine sugar cookie mix and hot cocoa mix. Add softened butter and egg, stirring until fully incorporated. Dust a clean, dry surface and a rolling pin with all-purpose flour. Place the cookie dough on the floured surface and roll out until ¼ inch thick. Using tip of a paring knife or a cookie cutter, cut dough into mug shapes. Place cookies onto an ungreased sheet pan and bake 10−12 minutes or until golden brown. Remove cookies from oven and allow to cool. Once fully cooled, decorate with frostings and marshmallows to resemble a mug of hot cocoa with whipped cream and marshmallows on top.