Preheat oven to 300° F.
Whisk together flour, salt, baking powder and rosemary in a bowl.
Mix together butter, honey and confectioners sugar in a large bowl with an electric mixer at low speed, then add flour mixture and mix until dough resembles coarse meal with some small (roughly pea-size) butter lumps. Gather dough into a ball and transfer to a lightly floured surface. Knead dough until it just comes together, about 8 times. Halve dough and form each half into a 5-inch disk.
Roll out 1 disk (keep remaining dough at room temperature) between 2 sheets of parchment into a 9-inch round (trim as necessary). Remove top sheet of parchment and transfer dough on bottom sheet of parchment to a baking sheet. Score dough into 8 wedges by pricking dotted lines with a fork, then mark edges decoratively. Arrange rosemary sprigs (if using) decoratively on top of dough, pressing lightly to help adhere, and sprinkle dough with 1/2 tablespoon granulated sugar.
Bake shortbread in middle of oven until golden brown, 20 to 25 minutes. Slide shortbread on parchment to a rack and cool 5 minutes. Transfer with a metal spatula to a cutting board and cut along score marks with a large heavy knife.
Make another shortbread with remaining dough.