Preheat oven to 350°F (175°C). Grease a 9-inch square baking pan. Line with parchment paper, and grease the paper.
Whisk together the flour, baking powder, and salt. Set aside.
Beat the melted butter and brown sugar until combined. Mix in the eggs and vanilla.
Gradually add the flour mixture, stirring just until combined. Stir in the hazelnuts and Nutella.
Transfer the dough to the prepared pan, and spread evenly into the bottom of the pan. Bake for 25 minutes, or until a pick inserted into the center comes out with a few moist crumbs.
Cool in the pan for at least 15 minutes before cutting into squares. If you like, top with a dollop of Nutella and more hazelnuts.
Notes
Store in an airtight container at room temperature up to 3 days.