In a small bowl, whisk together the gluten-free flour, granulated sugar, brown sugar, cinnamon, and salt.

Pour in the slightly cooled melted butter and mix with a fork until crumbs form. You will have some large pieces of streusel the size of a walnut halve and some smaller pieces.

Chill in the refrigerator while you continue with the recipe.

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 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 egg, sour cream, milk, and vanilla. Mix until smooth, scraping 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 ¾ cup of the mini chocolate chips to the batter, reserving the rest for later. Mix to combine.

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

Sprinkle the remaining 3/4 cup of the mini chocolate chips evenly over the batter.

Sprinkle the chilled streusel over the chocolate chips.

Bake at 350°F for 30-40 minutes, or until the cake is light golden brown and a toothpick inserted into the center comes out clean.

Let the coffee cake cool completely in the pan on a wire rack.

For the glaze, whisk together the powdered sugar, 1 tablespoon of milk, and vanilla extract in a small bowl. If the glaze is too thick, add more milk until it reaches your desired consistency.

Drizzle the glaze over the cooled coffee cake and allow it to set before serving.