How to Make It
Process granulated sugar, rosemary, and lavender buds in a food processor until rosemary and lavender are finely chopped, about 1 minute.
Beat butter on medium speed with a heavy-duty electric stand mixer until creamy. Gradually add sugar mixture, beating until smooth.
Stir together flour, baking powder, and salt in a medium bowl. Gradually add flour mixture to butter mixture, beating on low speed just until blended.
Shape dough into 2 (8-inch) logs. Roll each log in 1 tablespoon of the demerara sugar. Wrap each log in wax paper, and chill 3 hours.
Preheat oven to 350°F. Cut each log into 24 slices. Place slices 1 inch apart on baking sheets lined with parchment paper. Bake in preheated oven until edges of slices are golden, 10 to 12 minutes.
Transfer cookies to wire racks, and cool completely, about 20 minutes. Store in airtight containers.