In a medium mixing bowl, combine the gluten-free flour, almond flour, and salt. Set aside.
In a stand mixer fitted with the paddle attachment, beat together the butter and powdered sugar until light and fluffy – about 5 minutes. Make sure to scrape down the sides.
Add the whole egg and vanilla, then beat again until well-combined for an additional minute.
Add flour mixture to the bowl and beat until just combined and a crumbly looking dough has formed.
Remove dough from the stand mixer bowl and place it on a parchment-lined sheet tray. Form it into a disk shape or rectangular with your hands. If your dough is sticky, add a little sprinkle of additional flour. This should help with kneading it together.
Cover the sheet tray with foil or wrap in plastic and allow for it to rest in your fridge for 4hrs and up to 3 days.
Remove the dough from the fridge and allow it to sit at room temperature for 5 minutes.
Place dough onto a lightly floured surface, and roll the dough out to about ¼” thick. Try not to roll it too thin. I think it’s the easiest to do this on floured parchment paper.
Use a cookie cutter to cut into desired shapes. I use a 2 ½” scalloped edge cookie cutter for the tops of the cookies and a 2 ½” round cookie cutter for the bottom. You can also use the same cutter for tops and bottoms. Use a tiny cookie cutter (or the back end of a piping tip) to cut out the centers in your top cookies. You want to have an even number of tops and bottoms.
Combine all the dough scraps, roll them out again and cut out cookies. I do not recommend repeating this more than 2 times or your dough will become tough and start to spread while baking. One fun way to use dough scraps is to just bake them off as is and eat them as snacks.
Use an offset knife to transfer your cookies to a parchment-lined cookie sheet. Place the cookie sheet in the freezer for 15-20 minutes to allow the dough to firm up.
In the meantime preheat your oven to 350F.
When your oven is hot, arrange 8-10 cookies on a sheet tray and bake for 10-12 minutes. You want the edges lightly turning brown.
Remove from the oven and allow the cookies to cool 5-10 minutes before transferring to a cooling rack.
Repeat the baking with all the cookies you have cut out. Make sure to not use a hot sheet tray to bake them or they will spread out!
When the cookies are baked and COLD, dust your top cookies (with the cut-out centers) with powdered sugar and spread Nutella on the bottom cookies (withOUT the cut-out centers). Sandwich the two halves together.
Enjoy your cookies or store them in an airtight container at room temperature for up to 7 days. The cookies will become softer over the time of days.