Preheat the oven to 350°F then line a baking sheet with parchment paper or a silicone mat.
In a measuring cup or small bowl, combine melted coconut oil, maple syrup, and vanilla. Whisk until combined.
In a large bowl, add the oat flour, almond flour, tapioca starch, salt, and baking powder. Whisk together, making sure to break up any large lumps.
Pour the wet mixture over the dry mixture. Using a spatula, fold until it comes together as a dough. It might seem dry and crumbly at first but keep folding and eventually a moist dough will form. Shape the dough into a large ball, then break it apart so you are left with two even halves.
Place a nonstick mat or large piece of parchment paper on the counter and place one ball of dough in the middle. Using a rolling pin, roll out the dough until it is 1/8- to 1/4-inch thick. Cut the rolled-out dough into shapes using your desired cookie cutters. Carefully transfer each cookie onto the prepared baking sheet(s), placing each an inch apart. Repeat this process until you’ve used up all of the remaining dough. If you're having a hard time handling the dough you can try wrapping it and chilling in the refrigerator for 15-20 minutes. I've never had to do this but the temperature of your home might differ.
Bake the cookies for about 10 minutes, until their bottoms are very lightly goldened. Allow the cookies to cool completely on the baking sheet before moving. They will continue to firm up as they cool.
Prepare the Frosting
Meanwhile, prepare the frosting by combining the powdered sugar in a bowl with the milk. Keep stirring until a thick frosting forms. Transfer to a piping bag with a very small circular tip. Decorate cookies and top with sprinkles, as desired. Store leftovers in an airtight container in the refrigerator for up to 1 week.
Nutrition Facts are for one cookie without frosting since frosting amount will vary depending on how much you use.