Preheat oven to 350 degrees. Grease a 9x13 inch baking dish with butter or gluten-free cooking spray.
Mix 2 cups of flour, baking powder, sea salt and xanthan gum in a medium bowl with a whisk and set aside.
Beat the butter with the brown sugar and granulated sugar until light and fluffy. Add the eggs and beat until smooth.
Beat in half of the flour mixture until smooth, add the milk and the rest of the flour and beat until evenly incorporated.
Toss 1/2 cup of the blueberries with the remaining teaspoon of flour and gently fold into the batter. Spread into the prepared baking dish.
Mix the topping ingredients together and distribute evenly over the top of the batter.
Bake for 45-50 minutes or until a toothpick inserted into the middle of the cake comes out clean.