Sift together flours, salt and baking powder. Cream butter and sugar until just incorporated. Do not overmix!
Add the sifted dry ingredients to the butter mixture. Mix on low until just incorporated.
Roll the dough 1/2-inch thick (if the dough is too soft to roll, shape into a 4 x 10-inch rectangle, wrap in plastic and chill until firm). Cut into bars, circles or desired shapes and dock in the centesr with a fork. Wrap and chill until hard, 2 hours or overnight.
Preheat oven to 350°F. Line a baking sheet with parchment paper.
Place cookies on prepared baking sheet and sprinkle with sparkling sugar, if desired. For light coloring, do not separate bar cookies. Bake until edges are lightly browned, about 20 minutes.
Cool and store, wrapped, at room temperature for up to 1 week.