Mill your flour: Mill 144 grams hard white wheat berries and 96 grams soft white wheat berries. Whisk the freshly milled flours together until evenly combined to create a balanced flour blend that yields approximately 240 grams (2 cups) flour. If preferred, substitute 2 cups all-purpose flour.
Preheat the oven: Preheat to 350°F and line two baking sheets with parchment paper. This helps prevent sticking and makes cleanup easier.
Cream the butter and sugar: In a large mixing bowl, beat the softened butter and granulated sugar together for 3–4 minutes, until light and fluffy. The mixture should become noticeably lighter in both color and texture.
Add egg, sourdough discard, and vanilla: Mix until fully incorporated and smooth. Scrape down the sides of the bowl as needed to ensure everything blends evenly.
Add dry ingredients: Stir in the flour blend, baking powder, and salt just until a soft dough forms. Do not overmix once the flour has been added.
Chill the dough: Cover and refrigerate for at least 1 hour. Chilling helps the dough firm up and keeps the cookies from spreading too much.
Scoop the dough: Portion the dough into evenly sized balls, about 1½ tablespoons each, and place them about 2 inches apart on the prepared baking sheets.
Flatten the cookies: Gently press each dough ball into a thick round shape using the palm of your hand or the bottom of a glass.
Create the sand dollar design: Arrange 5 sliced almonds in a star pattern in the center of each cookie and gently press them into the dough.
Add the sand dollar holes: Using a clean kitchen straw, punch four small holes around the almond design and one hole near the bottom center. Clear the straw between cookies as needed.
Finish with sugar: Sprinkle the tops lightly with white sanding sugar for a soft shimmer and sandy appearance.
Bake: Bake for 9–11 minutes, until the edges are set and the bottoms are lightly golden. Avoid overbaking to keep the cookies tender.
Cool: Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.