In a large mixing bowl beat butter and cream cheese with an electric mixer on medium to high speed for 30 seconds. Add powdered sugar, baking powder, and salt. Beat until combined, scraping sides of bowl occasionally. Beat in egg and vanilla until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour.
Divide dough into five portions. Place each portion into separate bowls. Tint four of the portions different colors using paste food coloring. Knead each dough gently until it is uniformly tinted. Wrap each portion in plastic wrap or parchment paper. Chill dough about 1 hour or until easy to handle.
Preheat oven to 375 degrees F. On a lightly floured surface, roll each portion of dough until 1/8 inch thick. Using a fluted pastry wheel, cut dough into 1-inch-wide strips. Cut the dough strips into squares, diamonds, and triangles. Arrange dough pieces in groups of about 10 on an ungreased cookie sheet, overlapping the edges slightly to form shapes just larger than 3-inch mitten-shape cookie cutter. Cut out dough with mitten-shape cutter. (Reserve scraps, re-roll, and cut to make marble cutouts.)
Bake for 8 to 9 minutes or until edges are firm and bottoms are very light brown. Sprinkle cookies with clear edible glitter. Transfer to a wire rack and let cool. Makes about 18 cookies.