Place the flour in a small bowl and whisk in the sugar, cinnamon, and salt.
Work the room temperature butter into the dry ingredients using your fingertips, or a fork until it's well incorporated and crumbly. Set aside.
Make Cake
In a large bowl, whisk together the oat flour, baking powder, and salt.
In another large bowl, using a stand mixer or hand mixer, beat the sugar, butter, and egg together. Beat in the milk and flour in alternate additions, beginning with the milk and ending with the flour until well combined. Gently fold in the blueberries.
Spread the batter in a buttered 8x8 pan lined with a piece of parchment paper. Preheat the oven to 375℉. Sprinkle the topping evenly over the top of the batter and let it rest while the oven heats.
Bake the cake for about 45 minutes, until a tester comes out clean and the top is set.
Remove from the oven, let cool for 20 minutes on a wire rack, then cut and serve directly from the pan.
Notes
Use room temperature ingredients. This is a suggestion that carries over from gluten-free to gluten-baking. Bringing your ingredients to room temperature for a bit before you add them to your batter or dough will help them incorporate better, resulting in a better final product.
Rest the batter. Make sure to rest the cake batter while the oven heats. I find that this is just about the right amount of time for a gluten-free batter to hydrate.
Make ahead: You can prepare the cake the night before if you wish. Make the batter and place it in the pan - do NOT add the streusel. Cover the cake batter tightly with plastic and put it in the fridge overnight. The next morning, make and add the streusel, then bake for 55 to 60 minutes. The cake will need a slightly longer baking time because the batter and pan will be cold.
Serve from the pan. Streusel-topped cakes can be difficult to remove from the pan without all of the streusel falling off! Go ahead and slice and serve the cake straight from the pan.
Oat flour substitute: Substitute sorghum flour by weight, or use 2 cups of gluten-free all-purpose baking flour or all-purpose or whole wheat pastry flour.
Make dairy-free: Use your favorite non-dairy butter and non-dairy milk in place of the butter and milk.
Refined sugar-free: Swap coconut sugar for the granulated sugar.
Try other fruit. Use this blueberry coffee cake recipe as a jumping-off place to incorporate other berries, such as huckleberries, raspberries, or a combination.