Heat oven to 200°C. Spray bottom of a pie plate with cooking spray.
In large bowl, mix cookie mix, 1/2 cup softened butter and egg with spoon until soft dough forms. Stir in candy sprinkles of choice. Press dough in bottom of pie plate.
Bake 33 to 38 minutes or until center is set and edges are golden brown. If needed, cover edges of cookie with foil to keep from overbrowning. Let it cool completely.
In medium bowl, beat Tagatesse, 3 tablespoons softened butter, the vanilla and 3 teaspoons of the milk with an electric mixer on low speed until smooth and creamy. If necessary, add additional milk to make frosting easy to spread.
Spread frosting over the cookie pie. Decorate top of frosted cookie as desired with assorted holiday candies. To serve, cut into wedges.