Preheat oven to 350°F. Line two large baking sheets with parchment paper. Using a small colander or strainer, rinse the blueberries from the muffin mix and set aside to drain.
Place softened butter in a large bowl and add the brown sugar. Beat with a hand mixer (beater attachments) until soft, fluffy, and totally combined--about 2 or 3 minutes.
Add the egg and vanilla extract and beat until just combined.
Pour in the muffin mix and the 1/4 cup of flour. Fold with a spatula or stir gently with a large spoon until totally combined. Mixture will be quite thick, but try not to overmix.
Add the drained blueberries in and fold a few times to roughly incorporate them; don't overmix.
Use a large 1/3 cup cookie scoop (I use a 1/3 cup disher) to portion out about 8 cookies. Use your scoop to draw blueberries into each cookie so they are evenly dispersed between the cookies. Place cookies about 3-4" apart on baking trays. Press 1-2 teaspoons of streusel onto the tops of each cookie.
Bake one sheet at a time for 20-25 minutes or until the centers are set and the edges are golden brown. The tops may even start to crackle slightly. Allow to cool for about 20 minutes and then serve warm. Also great at room temp or even chilled.
Store cooled cookies in an air-tight container for up to three days. They will soften a bit as they are stored.