Using electric mixer, cream together softened butter and sugar.
Add egg and vanilla and continue to cream on high until light and fluffy.
Sift together flour, baking powder, and salt.
With mixer on low, slow add in dry ingredients.
Mix until just combined. DO NOT over mix.
Remove from bowl and hand knead a few times, adding in another TBSP or two of flour if dough seems too sticky.
Divide into two and wrap in waxed paper.
Refrigerate for one hour (or up to overnight).
Preheat oven to 350*
Remove dough from 'fridge and roll when still slightly chilled. Roll to 1/4" and cut with your favorite cookie cutter.
Place on Silpat or Parchment line baking sheet and place in freezer for 5-10 min.
Bake for 8-9 minutes, or until cookies just barely start to brown around bottom edges. DO NOT over bake!!
Cool on baking sheet for 2-3 minutes then transfer to rack to finish cooling.
Decorate with Royal Icing.