With a paddle attachment beat butter, cream cheese and egg yolk until smooth, reserving the white for later. Stir in flour and baking powder. Mix until stiff dough forms. Cover with plastic wrap and refrigerate for 1 hour. Lightly beat egg white and set aside.
Preheat oven to 350 degrees F.
On lightly floured surface, roll dough, 1/2 at a time, into a 15 by 9-inch rectangle. With sharp knife, cut dough into 3-inch squares. Place 3 inches apart on ungreased, parchment-lined cookie sheets.
Brush with egg white. Lightly press about 1 1/2 inches of wooden stick into bottom center of each dough square. With sharp knife, cut the dough diagonally from each corner to within 1/2-inch of center of each square. Sprinkle about 1 teaspoon red sugar over each square. Fold alternate corners of the square to the center to form pinwheel, overlapping the dough at the center pushing down gently to seal in center. Press one M & M in center of each pinwheel.