In a large bowl, cream the 1 cup unsalted butter with the ¾ cup granulated sugar and ¼ cup brown sugar. Add 3 large eggs and 1 tablespoon vanilla and mix until well combined. In a medium bowl, whisk together 3 cups oat flour, 2 teaspoons ground cinnamon, 2 teaspoons baking powder, and ¼ teaspoon fine sea salt. Beat the dry mix into the butter mixture until combined. Place the dough in the fridge for 20 minutes.
When ready to bake the cookies, line 2 baking sheets with parchment paper and preheat the oven to 350°F. In a bowl, stir together the ¾ teaspoon ground cinnamon and 3 tablespoons granulated sugar.
Scoop the dough into 1-tablespoon balls and roll each in the cinnamon sugar until coated. Place on the prepared baking sheets and lightly press down on each one.
Bake for 12-15 minutes until the edges are just set and very lightly browned.
Remove from the oven and use the back of a spoon to lightly flatten the tops if you like, then cool on the baking sheet for 5 minutes. Transfer to a wire rack to cool completely.