Spray bottom and corners of an 8” X 8” square cake pan with cooking spray (or use butter or oil). Only grease the bottom and corners, and set aside.
Preheat oven to 425°F
FOR THE STREUSEL
In medium bowl, whisk brown sugar, white sugar, cinnamon, and gluten free flour. Using a fork, stir in melted butter until mixture resembles coarse crumbs. There should be some larger pieces and some smaller, it doesn’t need to be equal sizes. Set aside.
FOR THE COFFEE CAKE
In a large mixing bowl, whisk eggs thoroughly, about 30 seconds.
Add granulated sugar, brown sugar, oil, buttermilk, and vanilla and whisk until smooth and creamy.
Add gluten free flour, baking soda, baking powder, cinnamon, and salt and whisk just until combined. There will be a few lumps and that’s OK, it is important you don't overmix.
Spread coffee cake batter in an even layer into prepared pan.
Sprinkle crumb topping evenly on top and press down gently.
Bake at 425 for 7 minutes, then, keeping the oven door closed, reduce temperature to 350 and bake an additional 30-35 minutes or until a wooden pick inserted in center comes out clean. Let cool 20 minutes before serving.