Mix together the flour, sugars, and salt in a small bowl. Pour melted butter over the mixture and mix together with a fork until little lumps of streusel start to form. Set aside while preparing the blueberry muffin cookies.
Cream the butter and sugar. Add egg and vanilla extract and beat until just combined. Add muffin mix and 1/4 cup of flour. Fold with a spatula. Mixture will be quite thick.
Add the drained blueberries in and fold a few times to roughly incorporate them; don’t overmix.
Use a 1/3 cup scoop to scoop out the cookies. Press 1-2 teaspoons of homemade streusel onto the tops of each cookie. Bake.