Cup of Yum
1. Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper.
2. In a large mixing bowl with an electric mixer, combine cookie mix, butter, and egg until a soft dough is formed.
3. If using, fold in sprinkles and mix until they are evenly incorporated.
4. Spread cookie dough into prepared pan.
5. Bake in preheated oven for 23-25 minutes.
6. Remove pan to wire rack and cool completely.
7. Once the cookies are cool, prepare the buttercream.

For the Buttercream

1. In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl with an electric mixer, cream butter on medium-high speed for about 6 minutes. With the mixer on low speed, slowly add in powdered sugar, and continue beating until well blended.
2. Add in vanilla, and 1 tablespoon of milk. Blend on low speed until moistened.
3. If needed, add additional milk 1 teaspoon at a time, until you reach the desired consistency. Beat at high speed until frosting is smooth and fluffy; about 5 more minutes.
4. If desired, use gel food dye to color the frosting.
5. Spread frosting onto cookie bars, or use a small biscuit cutter to cut the cookie bars into small circles. Pipe frosting onto cookies and top with festive sprinkles.