In a mixing bowl, cream together butter, granulated sugar for 3-4 minutes, until light and fluffy.
Add milk, vanilla, and salt and mix to combine.
Add in the gluten-free flour about half of a cup at a time, mixing in between, until the flour is incorporated.
Scoop cookie dough out onto a piece of parchment paper and pat it flat so that it's no more than an inch high. Freeze for about 30 minutes.
While the cookie dough is chilling, make the blueberry filling. Add fresh blueberries, sugar, lemon zest, lemon juice, cornstarch, cinnamon and salt to a bowl and stir to mix.
Preheat oven to 350F. Grease a mini muffin tin with cooking spray.
Press about a tablespoon of cookie dough into each well of a mini muffin pan. Press the cookie dough up the sides of the well and make sure there are no thin spots.
Spoon some blueberry filling into each tart shell, being sure not to overfill. You may have leftover blueberry filling which is okay.
Bake mini tarts at 350F for 22-26 minutes, until the edges of the cookie shell is golden brown. Cool to let the filling set. Remove from muffin tins and serve.