Preheat the oven to 350 °F. Prepare a large cookie sheet by lining with a silicone mat.
In a large bowl, cream together butter and sugar. Add egg, egg yolk and vanilla and mix until well combined. Add cinnamon, cream of tartar and salt and combine. Finally, add the oat flour and mix until fully incorporated.
In a shallow bowl, prepare the cinnamon sugar by mixing together the second amount of cinnamon and sugar.
Scoop the dough into 1.5-2 tablespoon balls and roll each in the cinnamon sugar until coated. Place on your prepared baking sheet and lightly squish down so they are a bit wider than tall.
Bake for 8-11 minutes until the edges are just set and very lightly browned. The centres will be quite soft.
Remove from the oven and use the back of a spoon to lightly flatten the tops and then sprinkle with additional cinnamon sugar if desired.
Let to cool on the baking sheet for 5 minutes before using a spatula to move cookies to a wire rack to cool completely.