Begin by placing the frozen blueberries in a microwave-safe bowl. Pop them in the microwave until they are defrosted, which should take about 1 to 2 minutes. Allow them to cool to room temperature; this should take about 3 to 5 minutes. Remember, do not drain the blueberries.

In a small bowl, whisk together the flour, baking powder, and salt until they are well combined. Take a large bowl and transfer the cooled blueberries along with their juice into it. Add the white sugar and vegetable oil, and give it a thorough whisking until everything is nicely blended. Next, incorporate the egg whites, lemon zest, and vanilla into the mix until everything is seamlessly combined. Now, it’s time to add the flour mixture in two portions, mixing gently after each addition until just combined.

You will want to cover this heavenly mixture and let it chill in the refrigerator for at least 2 hours. Once the time has passed, preheat your oven to 400 degrees F (200 degrees C) and line two baking sheets with parchment paper. Before getting to the baking part, prepare the coating by placing the sugars in separate bowls. Retrieve your dough from the fridge, scoop out about 1 1/2 tablespoons of dough, and shape it into a ball. Coat it first in the white sugar, then roll it in the powdered sugar for that lovely finish. Place the dough balls on the prepared sheets, ensuring they are spaced at least 1 1/2 inches apart. Bake for about 8 to 10 minutes until the cookies are just set, and don’t forget to switch the baking sheets halfway through! Once done, let them cool on the sheet for a few moments before transferring them to a wire rack to cool completely.