These are my all-time favorite brownies - fudgy, rich, moist, and perfect.
Line bottom and sides of a 9 x 13 inch rectangular pan with foil or parchment paper. Preheat oven to 350 degrees.
Using a mixer, beat eggs, cocoa, salt, baking powder, espresso powder, and vanilla. Mix until smooth.
Place butter in microwave safe bowl and heat until melted, about 45-50 seconds.
Add sugar to melted butter and stir until well combined.
Add the butter/sugar mixture to the chocolate mixture and stir until combined.
Add flour, chocolate chips, and hazelnuts (if using) and stir gently until no more flour is visible
Place half the brownie batter in the prepared pan, distributing evenly. Using half of the dulce de leche, drop spoonfuls onto the brownie batter and swirl slightly with butter knife or small spatula. Add remaining chocolate mixture, spreading evenly and then top with the rest of the dulce de leche. Swirl them together gently with a butter knife.
Bake for about 30-35 minutes or until toothpick inserted in center comes out with just a few moist crumbs.
Allow to cool on wire rack before cutting into squares
These brownies are delicious without the addition of the nuts and dulce de leche too if you prefer a plain brownie.