Preheat oven to 375 degrees F.
Put the oats into a food processor and pulse until finely chopped. Add flour, salt, pepper, rosemary, baking powder, and butter and pulse until mixture resembles coarse meal. Add milk and pulse until a dough forms about 15 seconds. On a lightly floured surface, roll out dough to 1/8-inch thick (about a 13-inch round) and cut out about 60 square oatcakes. Arrange oatcakes on baking sheets 1-inch apart and bake in middle of oven 12 to 15 minutes, or until lightly brown on the bottom. Transfer oatcakes to a rack and cool completely.