Preheat the oven to 375 degrees and grease a 9 x 13 inch pan.
Melt 1 cup of butter or coconut oil. Let it cool, then divide it into a 3/4 cup portion for the base and a 1/4 cup portion for the crumb topping.
To make the base, combine the dry ingredients in a medium bowl. (Substitute white whole wheat flour for the gluten-free flour if you’re not gluten-free.)
Stir the base layer's wet ingredients together in a small bowl and then mix them into the dry ingredients.
Press the base layer mixture evenly into the pan.
In a medium bowl, stir together the blueberries, maple syrup, and lemon zest. Place this blueberry mixture in an even layer on top of the base.
To make the crumb topping, stir together the ingredients. Add this crumb topping mixture on top of the blueberry layer.
Bake for 30 to 35 minutes, or until the top is brown. Let it cool completely before cutting. If you’re serving these as part of a buffet, you can cut them quite small (24 pieces). Cut them into larger pieces for a stand-alone snack.