Preheat the oven to 300. Butter a 9x9 pan and line the
bottom and 2 sides of the pan with parchment paper.

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

In a large pot over medium heat melt butter stirring
occasionally.

Remove pan from the heat and whisk in brown sugar and vanilla extract.

Using an electric mixer whip in eggs. Mix on low to incorporate and then switch to medium high and whip 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 dry ingredients to the butter mixture and continue to mix until well mixed and no lumps remain.

Next evenly spread batter in the 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 if using a glass pan or 20-25 if
using a metal one. The edges will be golden brown and the middle still slightly gooey. Cooking time will vary depending on your pan so pay close attention while baking!

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.

Keep in an airtight container.