1. In the bowl of a stand mixer, cream together butter and sugar until light and fluffy. Add lemon zest, vanilla and egg and beat until incorporated.

2. In a medium bowl, whisk together both kinds of flour, salt and rosemary.

3. With the mixer on low, gently add dry ingredients into butter mixture. Mix until ingredients are well combined and a stiff dough is formed.

4. Cut two 12x16 inch pieces of plastic wrap. Divide dough in half placing one half of the dough on each piece of plastic wrap. Use your hands to form two 1 ½ inch diameter logs. Open the plastic wrap and sprinkle logs with sanding sugar (if using) and roll a bit more to coat the logs well in the sugar. Wrap logs tightly in the plastic wrap, place on baking sheet and freeze logs for 1 hour or until firm.

5. Preheat oven to 375°. Line baking sheets with parchment paper and set aside.

6. Remove dough logs from the freezer and remove the plastic wrap. Slice dough into ¼ inch thick slices and place on prepared baking sheets 1 inch apart.

7. Bake until cookies are just golden at the edges, about 16-18 minutes. Allow cookies to cool on wire racks. Store cookies in an air tight container for up to 4 days.