Beat the butter and sugar together in a bowl until creamy and smooth, and stir in the egg until well incorporated. Stir in the whole wheat flour, all-purpose flour, rosemary, and baking powder until well blended. Cut the dough into 2 equal-sized pieces, and shape each piece into a log about 1 ¼-inch in diameter. Wrap the logs in plastic wrap, and refrigerate at least 2 hours, or place in freezer for about 1 hour.
Preheat an oven to 350 degrees F (175 degrees C). Line baking sheets with parchment paper.
Cut the logs of dough into thin slices, ⅛ to ¼-inch thick. Place the slices on the prepared baking sheets, and bake in the preheated oven until the cookies are set and the edges turn golden brown, 10 to 12 minutes. Cool for 1 minute on baking sheets before removing to wire racks to finish cooling.