1 cup (2 sticks) butter
2 1/4 cups sugar
1/2 cup Nutella
4 large eggs
1 1/4 cups dark cocoa powder
1 teaspoon salt
1 teaspoon baking powder
1 teaspoon espresso powder
1 tablespoon vanilla extract
1 1/2 cups all-purpose flower
1 1/2 cups of chopped dark chocolate
about 1/4 additional Nutella for swirling (optional)
Pre-heat oven to 350 degrees F. Butter a 9x13 baking pan. In small saucepan over medium heat, melt butter completely. Stir in sugar and continue cooking for 1-2 minutes, stirring constantly. Remove from the heat and stir in the Nutella.
Pour butter mixture into a large bowl or stand mixer. Beat in cocoa powder, eggs, salt baking powder, espresso powder, and vanilla extract. Mix until well combined. Gently stir in the flour and baking melts until just combined. Spread into prepared pan. Add spoonfuls of Nutella on top, if using, and use a toothpick to swirl.
Bake for about 30-35 minutes, until a toothpick comes out mostly clean.