In a heavy bottom saucepan melt the butter on medium heat till it smells nutty and turns brown in colour. (Keep stirring constantly for about 5 minutes)Set aside to cool.
Preheat oven to 350°F, grease and line a 9x13-inch baking dish with parchment paper, leaving a 2-inch overhang on two opposite sides to easily remove the cookie bars after they are baked.
Add the brown butter, granulated sugar, and brown sugar in a large bowl and use a wooden spoon or mixer to stir/mix until well-combined
Mix the warm water and coffee, add it to the batter and mix well.
Mix in the eggs, and vanilla until completely combined.
Add the flour, baking powder, and salt, and mix until just combined.
Fold in chocolate chips and then spread the batter into the prepared pan, sprinkle extra chocolate chips on top if you like.
Dollop the Nutella on the spread batter and swirl it using a spatula or butter knife.
Bake until a toothpick inserted in the center comes out with just a few crumbs, 20 to 25 minutes.(The edges should look firm and well-baked; the center should be moist but not gooey.)