Beat the butter and sugar together in a bowl until creamy and smooth; stir in the egg until well incorporated. Stir in the wholemeal flour, plain flour, rosemary and baking powder until well blended. Cut the dough into 2 equal-sized pieces and shape each piece into a log about 3cm in diameter. Wrap the logs in cling film and refrigerate at least 2 hours or place in freezer for about 1 hour.
Preheat an oven to 180 C / Gas 4. Line baking trays with parchment paper or lightly grease.
Cut the logs of dough into thin slices, 3-6mm thick. Place the slices on the prepared baking trays 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 trays before removing to wire racks to finish cooling.