Preheat the oven to 350°F and line three baking sheets with parchment paper and set aside.
In a medium bowl, cream together the butter, sugar, egg, and vanilla until light and fluffy.
In a separate bowl, whisk together the flour and baking powder.
Add the flour mixture to the butter mixture and beat just until combined.
Shape the dough into 3 different-sized balls of cookie dough. Use a rounded 1 teaspoon of dough to make small balls. Then you will use 2 rounded teaspoons of dough to make medium balls. And 3 rounded teaspoons of dough to make large balls. You should be able to make at least 15 balls of each size.
Place each size of balls on its own prepared baking sheet and use the bottle of a glass to gently flatten them. The different sizes of cookies should bake on different sheets as they will not bake at the same rate.
Bake each sized cookie separately until the bottoms are just golden. Small cookies will take 4 to 6 minutes. Medium will take 6 to 8 minutes. Large will take 8 to 10 minutes.
Transfer the cookies to wire racks to cool completely.
While the cookies cool, prepare the frosting by creaming together the powdered sugar and butter. Then beat in the milk, vanilla, and food coloring.
Use the frosting to “glue” together one of each sized cookie to make a Christmas tree. Pipe icing onto the top of the largest size cookie and place the middle size cookie on top. Pipe icing on the top of the middle size cookie and place the small size cookie on top. Then continue decorating with the icing and sprinkles.