Cream the butter and sugar together in the bowl of a stand mixer. Beat in eggs and vanilla. Stir in flour, baking powder and salt. Cover and chill the dough for at least one hour to overnight.
Preheat oven to 400 degrees. Roll out the dough on a floured surface 1/4-1/2 inch thick. Cut into circles with cookie cutters or a small cup. Place cookies 1 inch apart on a parchment lined baking sheet.
Bake 6-8 minutes. Cool completely before decorating.
Beat the butter in a stand mixer until light and fluffy. Add the vanilla and beat again. Gradually add the powdered sugar a little at a time. Add the milk a little at a time if its too thick.
Santa Cookie Assembly
Color half of the buttercream red. Add the white buttercream to a piping bag with a star tip.
Use a butter knife or small spatula to spread red frosting on the top 1/3 of the cookie.
Fill a piping bag with white frosting and a medium sized star tip. Pipe a white line just below the bottom of the red frosting.
Pipe a large star on the right side of the cookie, just above the white line (the tip of Santa’s hat)
Pipe two dots for Santa’s eyes and two curves for his mustache.
Place two green M&M’s on the white frosting dots for Santa’s eyes.
Put a red M&M just above the mustache for his nose.