In a medium bowl, whisk together the gluten-free flour, cocoa powder, baking soda and salt.
In the bowl of a stand mixer fitted with a paddle attachment, or using a hand mixer, beat the butter and Nutella on medium-high speed for 1 minute until creamy.
Add the brown sugar and granulated sugar and mix until creamed, about 2-3 minutes.
Add the egg and vanilla and mix until well-combined. Scrape down the sides and bottom of the bowl.
With the mixer running on low, slowly add the dry ingredients to the wet ingredients and mix until well-combined. Scrape down the sides and bottom of the bowl, then mix again until fully incorporated.
Cover the bowl and let sit at room temperature for 20.
Preheat the oven to 350°F. Line large baking sheets with parchment paper or silicone baking mats.
Place the unwrapped chocolate Kiss candies in a bowl and set them in the freezer.
Scoop the cookie dough into round balls, 1 tablespoon per ball, then roll in granulated sugar, if using. (Don’t make them too big or you won’t have the right cookie-to-chocolate ratio.)
Place on the prepared baking sheets, about 2 inches apart. Bake for 10-12 minutes, until the tops begin to slightly crack.
Remove from the oven and immediately press a frozen chocolate Kiss candy into each cookie.
Cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.