Preheat the oven to 350F and coat a 9x5-inch loaf pan with gluten-free nonstick cooking spray, then line with parchment paper.
In a large bowl, whisk together the gluten-free flour blend, xanthan gum, cocoa powder, granulated sugar, baking powder, baking soda, and salt. Stir in the chopped hazelnuts and set aside.
In a medium bowl, whisk together the eggs, brown sugar, yogurt, milk, oil, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined. Transfer the batter to the prepared loaf pan and spread evenly in the pan.
Use a spoon to dollop the chocolate hazelnut spread over the top of the batter. Use a butter knife to swirl it in, then top with the remaining 1 tablespoon of chopped hazelnuts.
Bake for 50-60 minutes or until a cake tester or toothpick comes out clean. Cool in the pan for 20 minutes on a wire rack, then remove from the pan and parchment paper to cool completely.
Store the room temperature bread wrapped tightly in plastic wrap for up to 3 days. To freeze, wrap tightly in plastic wrap and keep in a freezer-safe bag for up to 3 months.
Notes
*For dairy-free, use unsweetened coconut milk beverage, dairy-free plain (or vanilla) yogurt, and dairy-free chocolate hazelnut spread. I prefer Nocciolata.