In a medium bowl, whisk together the almond flour, cornstarch, baking powder, baking soda, and salt. Set aside.

In a large bowl whisk together the melted butter, granulated sugar, and brown sugar until combined. Add the egg, vanilla, and almond extract. Whisk until combined.

Add the dry ingredients to the wet ingredients and mix with a silicone spatula or wooden spoon until fully combined. Fold in the sprinkles, if using. The dough will be very soft. Cover and chill the dough for at least 2 hours (or up to 3 days). Do not skip chilling.

Preheat the oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats. Have ready a small bowl of granulated sugar for coating the cookie dough balls.

Scoop and roll dough into balls, about 1.5 Tablespoons (28–30g) of cookie dough each. Roll the dough balls in the sugar, and arrange 3 inches apart on the lined baking sheets. Using the bottom of a measuring cup, gently press down on each cookie to slightly flatten.

Bake the cookies for 13–15 minutes, until the surface looks crackly and the edges are set but the centers still look soft. Allow cookies to cool on the baking sheet for 10–20 minutes and then transfer to a cooling rack to cool completely.

If desired, decorate cooled cookies with buttercream frosting and top with sprinkles. For the pictured frosted cookies, I used the Wilton 12 piping tip to swirl buttercream on top of the cooled cookies.

Store unfrosted cookies in an airtight container at room temperature for up to 5 days. Store frosted cookies in an airtight container at room temperature for up to 1 day, or in the refrigerator for up to 5 days.