Preheat your oven to 350°F.
Line an 8x8-inch baking pan with parchment paper.
In a large mixing bowl, combine the cookie mix, butter, and egg. Use an electric mixer on medium speed until a soft dough forms.
Fold in the festive sprinkles if using.
Spread the cookie dough into the prepared pan evenly.
Bake for 23 to 25 minutes.
Remove from the oven and cool completely on a wire rack.
In a stand mixer, cream softened butter for about 6 minutes.
Gradually add in the powdered sugar until well blended.
Add in the vanilla and 1 tablespoon of milk. Adjust consistency with more milk if needed.
If desired, color your frosting with gel food dye.
Once cookies are cool, frost and decorate as desired.
