Preheat oven to 180℃/350°F. Line an 8-by-8-inch baking pan with parchment paper and set aside.
Melt the butter. Once slightly cooled, add in sugars and whisk. Add the egg and vanilla and stir vigorously to combine.
1/2 cup butter, 3/4 cups dark brown sugar, 1/4 cup white sugar, 1 egg, 1 tbsp vanilla
Add the flour, baking soda, and salt and fold until just incorporated. Don't overmix!
1 1/2 cups all-purpose flour, 1/2 tsp baking soda, 1/4 tsp salt
Place ¾ of the cookie dough on the bottom of your prepared pan. Make sure it's covered the entire bottom of the pan.
Dollop Nutella on top. It doesn't have to be an even layer. Pour sweetened condensed milk over the top. Add the remaining dough on top in irregular pieces. Some Nutella and condensed milk can be showing.
1/2 cup sweetened condensed milk, 1/2 cup Nutella
Bake for about 30 minutes, or until a toothpick inserted into the center comes out with no cookie dough. The edges will be firm and golden. The sweetened condensed milk could be a little jiggly, but it firms as it cools.
Allow the bars to completely cool. As they cool, they set. If you slice into them while they're still warm, the bars will fall apart. You can speed up the cooling process by placing them in the refrigerator.