Make the caramel sauce:

Warm brown sugar over medium heat in a non stick pan until it starts to melt, stirring constantly

Lowe heat and add butter, 1 T at a time, continuing to stir constantly

Remove from heat, add cream and salt 

Stir until smooth then let cool slightly

For the cookies:

Cream together butter and sugar until light and creamy

Add eggs and vanilla, stir until smooth

Add flour and salt, mixing slowly until just combined

Gently fold in the caramel sauce until swirled through the batter

Line a 7″ x 10″ baking pan with parchment paper

Pour 1/2 of the batter in the pan, smoothing it out

Drop the Nutella onto the batter in a single layer, then spread it out 

Cover with the other half of the batter

Bake at 350 degrees for about 25 minutes or until a toothpick comes out clean

Wait 10 minutes, then transfer to a wire rack

Cut into squares while still slightly warm