Line a small baking sheet or plate with parchment paper or wax paper. Dollop or pipe the ½ cup Nutella into small ½ inch circles, until you get 12 total. Place in the freezer for at least an hour.
Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats. Set aside. In a medium mixing bowl, combine 1¼ cups gluten-free flour blend, 1½ teaspoons baking powder, and ½ teaspoon baking soda. Whisk to combine. Set aside.
In a large mixing bowl, add the ½ cup unsalted butter, ½ cup creamy peanut butter, ¾ cup light brown sugar, and ¼ cup granulated sugar.
Cream with an electric mixer for 2-3 minutes, until light and fluffy, scraping down the sides of the bowl with a spatula as needed.
Add the 2 teaspoons pure vanilla extract, 1 large egg, and ½ teaspoon salt. Mix until smooth.
Add half of the flour mixture to the butter mixture and stir until just combined.
Repeat with the remaining flour, stirring until no more streaks of flour remain.
Stir in the 4 ounces semi-sweet chocolate bar chunks until combined.
Using a 2-3 tablespoon cookie dough scoop, fill the scoop halfway full with cookie dough. Then, press a frozen Nutella chunk into the cookie dough until it is submerged. Fill the scoop the remaining way with cookie dough so that the Nutella is completely covered.
Place dough on the prepared baking sheet and repeat with remaining cookie dough and Nutella.
Bake at 350°F (175°C) for 10-12 minutes, until the edges are just set.
Remove from the oven and immediately sprinkle with 1 teaspoon flaky sea salt. Let cool before serving.