In a dry skillet, toast the hazelnuts over medium heat, stirring or tossing them frequently. Toast for 5-6 minutes or until they start to turn golden brown and become fragrant. Remove the pecans from the pan immediately and transfer them into a bowl. Rub the hazelnuts between your hands to loosen and remove the skins as much as possible. It's okay if they don't come off completely (see notes for more tips if you're sensitive to the skins).
Transfer the sorta-skinned hazelnuts to a food processor. Pulse the food processor until the hazelnuts form a very fine crumb.
In a mixing bowl, stir together the oats, gluten-free flour, cocoa powder, coconut sugar, baking soda, salt, and cinnamon. Transfer the dry mixture to the food processor and pulse to combine.
Add the maple syrup, nondairy milk, coconut oil, and vanilla extract, and blend until very smooth, scraping down the sides with a spatula as needed. Scrape the mixture out of the food processor and return it to the mixing bowl; stir in the chocolate chips. I find that a strong fork is the best way to get the chocolate chips evenly mixed in, since the dough is quite thick.
Chill the dough in the refrigerator for at least 30 minutes. Preheat the oven to 350 degrees Fahrenheit and lightly grease two baking sheets.
Take about 1 and ½ tablespoons of dough at a time (or more or less, depending on what size you want your cookies to be), roll into a ball, then flatten it as you place it on the baking sheet. These cookies only tend to spread a little bit, but do puff up a little bit, so keep this in mind when spacing out the dough balls. This dough can be very sticky. You may need to stop to clean your hands partway through.
Bake for 9-11 minutes or until the edges of the cookies are just starting to feel crisp. Take care not to overcook - they can dry out easily. If using two baking sheets, switch their positions halfway through baking.
For slightly flatter, chewier cookies, press down on each of the cookies with a fork right when they come out of the oven. Let the cookies cool for a few minutes before carefully removing them with a spatula (they may stick if you try to pick them up directly).
Leftover cookies keep in an airtight container at room temperature for a couple of days. Unused raw cookie dough can be kept in the refrigerator or freezer and used later. If frozen, thaw in the refrigerator before baking.