Microwave the frozen blueberries for 30-60 seconds until they are soft enough to mash, then mash with a fork until all the berries are mashed. Place in the fridge until the texture is less watery and thicker like jam (10-20 minutes)
Preheat oven to 350°F and line 2 baking sheets with parchment paper.
In the bowl of a standing mixer with the paddle attachment or a handheld mixer, combine butter and sugars, mixing until light and fluffy (1-2 minutes).
Add in the blueberries and beat well until combined. Add in the egg and vanilla and mix again.
Mix in the dry ingredients (flour, salt, and baking powder) until a dough forms.
Use a medium cookie scoop or spoon to make 1.5-2 tbsp sized balls and place them on the baking sheet around 3-4 inches apart.
Bake the cookies (one sheet at a time for even baking) for 8-11 minutes. Remove them from the oven when the edges look set. They will look under-baked in the middle! Remove from the oven and leave them on the baking sheet for 5 minutes, then transfer to a cooling rack to cool completely. Enjoy!
Video
Notes
Measuring Flour: Be sure to properly measure your flour using either a kitchen scale or by fluffing it up in the bag with a fork, then using a spoon to scoop the flour into the measuring cup and then use the back of a butterknife to level it off (the is the scoop and level method)Blueberries: Be sure to use flavorful blueberries, or else the blueberry flavor in the cookies won't be strong. Storage and Freezing:Store the cookies in an airtight container or ziploc bag at room temperature for 5-6 days.The baked cookies can be frozen in a freezer bag for up to 2 months. Thaw at room temperature when you're ready to eat them.