In a large mixing bowl
, beat butter and shortening with an electric mixer for 30 seconds. Add sugar, baking powder, and salt; beat until combined. Beat in egg, milk, and vanilla until combined.
Beat in as much of the flour
as you can with the mixer. Stir in any remaining flour. Divide dough in half. Cover; chill dough about 1 hour or until easy to handle.
Preheat oven to 375 degrees F. On a lightly floured surface, roll half of the dough at a time to 1/8- to 1/4-inch thickness. Using desired tree-shaped cookie cutters, cut out dough.
Place about 1 inch apart on ungreased cookie sheets
. Bake for 7 to 9 minutes or until the edges are firm and bottoms are very lightly browned.
Transfer to a wire racks and let cool. Frost with Royal Icing and decorate with sprinkles.