Shortbread Cookies with Blueberry Sparkling Sugar

To prepare sparkling sugar: Preheat oven to 200 degrees F. Line a rimmed baking sheet with parchment paper.

Whisk decorating (or sanding) sugar and blueberry powder in a medium bowl. Add water and stir until the sugar is evenly coated. Spread into an even layer on the prepared pan. Place in the oven and turn off the heat. Let the sugar dry for at least 30 minutes and up to 2 hours. (If using right away, the sugar doesn't have to be completely dry.) If there are clumps, lay a sheet of parchment on top of the sugar and use a rolling pin to crush.

Meanwhile, prepare cookies: Whisk all-purpose flour, whole-wheat flour, baking powder and salt in a medium bowl. Beat butter, oil, sugar and vanilla in a large bowl with an electric mixer until smooth, 1 to 2 minutes. Gradually beat the dry ingredients into the wet ingredients on low speed until evenly incorporated. If the dough does not come together, add water, 1 tablespoon at a time until it holds together.

Transfer the dough to a large sheet of parchment paper and shape into a rough square. Cover with another sheet of parchment and roll out into a 12-inch square. Check the underside and roll out any wrinkles. Place the dough, with the parchment, on a large cutting board and refrigerate.

When the sugar is done, increase oven temperature to 325 degrees . Line a large baking sheet with parchment paper.

Peel the top sheet of parchment off the dough and set aside. With a large knife, carefully trim the dough to form an even square; discard trimmings. Sprinkle the dough evenly with the blueberry sparkling sugar. Replace the parchment over the sugar and press down to embed the sugar in the dough. Discard the top sheet and cut the dough into 36 squares. Using a spatula, transfer the cookies to the prepared pan, spacing them about 1/2 inch apart.

Bake the cookies for 10 minutes. Reduce oven temperature to 250 degrees . Continue baking until the cookies are lightly browned on the bottom, 10 to 12 minutes more. Let cool on the pan for 10 minutes, then transfer to wire racks to cool completely.