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.