Preheat the oven to 350°F and grease an 8×8-inch square baking pan with cooking spray or line with parchment and set aside.

To prepare the streusel topping, mix together the gluten-free flour blend, granulated sugar, brown sugar, cinnamon, and salt in a medium bowl until combined. Add the melted, but cooled, butter into the dry ingredients. Mix with a fork until everything is combined. You’ll have varying sizes of pieces of streusel, which is what you want. Place the streusel in the refrigerator until ready to use.

Place the blueberries in a small mixing bowl and top with 1 tablespoon of gluten-free flour. Toss until the blueberries are coated.

To make the cake, add the remaining gluten-free flour blend, baking powder, baking soda, and salt to a large bowl. Whisk until combined.Set aside.

Place the butter and granulated sugar in a separate mixing bowl. Use an electric mixer and beat until light and fluffy, about 2-3 minutes.

Add the eggs, sour cream, milk and vanilla. Mix until smooth. Scrape down the sides of the bowl with a spatula as needed.

Add the dry ingredients to the wet ingredients and mix until just combined and no more streaks of flour remain.

Add the flour covered blueberries to the batter. Use a spatula to gently mix the blueberries into the batter, being careful not to break any of them.

Spoon the coffee cake batter into the prepared baking dish. Smooth with a spatula.

Sprinkle the chilled gluten-free streusel over the batter.

Bake at 350F for 43-48 minutes, or until the cake is a light golden brown and when a toothpick is inserted into the center it comes out clean.

Let the coffee cake cool completely.

Prepare the glaze by adding the powdered sugar, 1 tablespoon of the milk, and vanilla extract to a small bowl. Whisk until combined. If the glaze is too thick add more milk until it reaches your desired consistency.

Drizzle the glaze over the coffee cake and allow to set before serving. Slice and enjoy!