Make the dough: Place rolled oats in a food processor, and process until finely ground. In a medium bowl, whisk together the ground oats, baking soda, cream of tartar and cinnamon.
In a large bowl, mix melted coconut oil, sugar, vanilla extract and milk. Using a hand mixer or a stand mixer fitted with a paddle attachment, beat on high speed until smooth and creamy, about 2 minutes.
Slowly add the dry ingredients to the wet ingredients. The batter should be thick, and not too sticky.
Cover the bowl with cling foil, and let the batter rest in the fridge for about an hour.
Make the topping: Combine granulated sugar and cinnamon together in a small bowl.
Preheat the oven to 375°F (190°C). Line two large cookie sheets with parchment paper. Set aside.
Remove the bowl from the fridge. Roll cookie dough into balls, about 1 ½ tablespoons of cookie dough each. Roll the dough balls in cinnamon-sugar topping. Arrange the cookies 3 inches apart on the baking sheets.
Bake for 9 to 10 minutes. When they are still very warm and soft, lightly press down on them with the back of a spoon to help flatten them out. Allow cookies to cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely.