In a large mixing bowl on medium speed, cream together butter and sugar until smooth. Add eggs (one at a time) and vanilla until thoroughly mixed. Add flour (one cup at a time), then add baking powder and salt.
Cover, and chill dough for at least one hour (or overnight).
Preheat oven to 400 degrees F.
Roll out dough onto a floured surface, 1/4 to 1/2 inch thick. Cut into shapes with any cookie cutter. Place cookies 1 inch apart on parchment lined cookie sheets.
Bake 6 to 8 minutes in preheated oven. Cool completely.
My cookies baked perfectly at 6 minutes. For purple sugar cookies, add red food coloring to your sugar and mix in a few drops of blue. Keep adding color until you get to desired color.