Line a baking sheet with a piece of parchment paper. Set aside.
In a medium mixing bowl, whisk together the almond flour, arrowroot flour, tapioca flour, coconut flour, baking powder, salt, and milk powder (if using). Tip - Milk powder adds creamy flavor, makes cookies chewy, and delicate crust.
In a medium mixing bowl, add the butter and sugar. Use a hand held mixer fitted with the whisk attachment and beat on medium-high speed until fluffy and creamy - about 3 - 5 minutes. Tip - You can also use your stand mixer fitted with the whisk attachment along with 3 quart mixing bowl that fits a standard tilt-head stand mixer.
Add the egg white and vanilla extract to the butter mixture and continue to beat on medium-high speed until the mix is light and creamy - about another 3 minutes.
Add the sprinkles (if using), and stir in by hand with a spatula. Using a cookie scoop (I use a #40 cookie scoop), and scoop the cookie dough into 6 equal portions. Place about 2 inches apart on the prepared baking sheet. Cover the baking sheet with plastic wrap and refrigerate the cookie dough for at least 30 minutes. You can even chill overnight.
As the cookies are chilling, preheat the oven to 350F degrees, and adjust the oven rack to the middle position.
Bake the chilled cookies for 14 - 16 minutes, or until the edges are slightly golden. Tip - For a crispier cookie, bake for about 1 minute longer. For a chewier cookie, bake for about 1 minute less.
Cool the cookies on the baking sheet set on a wire cooling rack for 5 minutes. Then transfer the cookies to the wire rack to cool completely. Tip - For a perfectly round cookie - while the cookies are still warm on the baking sheet, use a round biscuit (or cookie) cutter slightly larger than the cookie, and swirl the cookies around the inside of the cutter.