Preheat oven to 350 degrees.
Combine butter and cream cheese with an electric mixer.
Add sugar and mix well.
Mix in the egg and the vanilla.
Add in the flour, baking powder, and salt.
Mix well until the dough forms.
Fold in holiday sprinkles (I used about ½ cup. You could do more or less).
Line a 9x13 baking dish with parchment paper and lightly spray with cooking spray.
Bake for 30 - 35 minutes (time may depend on how thick your dough is).
Remove from oven and let cool completely.
While the cookie is cooling, make the frosting (directions below).
Once the cookie is cooled, remove from baking dish and frost.
Garnish with holiday sprinkles and cut into squares or bars.
Cream together softened butter and vanilla.
Add in powdered sugar and salt.
Mix well with a fork or an electric mixer.