Preheat the oven to 300℉. Butter a 9×9 pan and line the bottom and 2 sides with parchment leaving an inch or two overhang on each side.

In a medium size bowl mix together flour, baking powder and salt, set aside.

In a large pot over medium heat brown the butter stirring occasionally.

Remove from heat, whisk in brown sugar and vanilla.

Using an electric mixer whip in eggs. Mix on medium high until light and fluffy. It is important that this mixture is well mixed and the color changes from a darker brown to a lighter tan.

Add in the dry ingredients and continue to mix until well mixed and no lumps remain.

Evenly spread batter in prepared pan.

Dollop hazelnut spread by the spoonful evenly across the surface of the batter. Drag a butter knife through the spread to swirl it through the batter. Drag it through again in the opposite direction to swirl more.

Bake for 30-35 minutes in a glass pan, or for 20-25 minutes if using a metal one. The edges will be golden brown and the middle still slightly gooey.

Remove from the oven and allow to cool at least 2 hours before cutting. It is normal for the middle to sink below the edges.

Cut into 16 squares to serve.