Ingredients

• 4 ounce unsalted butter
• 6 ounce unsweetened chocolate, chopped
• 2 ounce semi-sweet chocolate, chopped
• 1 cup granulated sugar
• 0.5 cup packed light brown sugar, packed
• 3 eggs
• 1 teaspoon vanilla extract
• 2 teaspoon instant espresso powder
• 0.75 cup all-purpose flour
• 0.5 teaspoon baking soda
• 0.5 teaspoon kosher salt
• 0.25 teaspoon fleur de sel

Steps

1. Preheat oven to 350°F. Line a 9x13 inch baking pan with parchment paper, allowing overhang on two sides for easy removal.
2. In a medium saucepan over low heat, combine unsalted butter, unsweetened chocolate, and semi-sweet chocolate. Stir frequently until completely melted and smooth, about 3-4 minutes. Do not let the mixture exceed 115°F or the chocolate may seize.
3. Remove the saucepan from heat and let cool for 2 minutes.
4. In a mixing bowl, whisk together granulated sugar and brown sugar until combined.
5. Add eggs to the sugar mixture one at a time, whisking well after each addition until the mixture is thick and glossy, about 2 minutes total.
6. Whisk vanilla extract into the egg mixture.
7. Pour the cooled chocolate mixture into the egg mixture and whisk until fully combined.
8. Dissolve espresso powder in 1 teaspoon of warm water, then whisk into the chocolate batter until fully incorporated.
9. In a separate bowl, whisk together flour, baking soda, and kosher salt.
10. Fold the dry ingredients into the chocolate mixture using a rubber spatula, stirring until just combined. Do not overmix; a few small streaks of flour are acceptable.
11. Pour the batter into the prepared baking pan and smooth the top with a spatula.
12. Bake for 25-28 minutes until a toothpick inserted in the center comes out with a few moist crumbs but no raw batter. The edges will pull slightly away from the pan.
13. Remove from the oven and immediately sprinkle the fleur de sel evenly over the warm brownie surface.
14. Let the brownies cool in the pan for 10 minutes, then lift out using the parchment overhang and transfer to a wire rack to cool completely, about 20 minutes.
15. Once completely cooled, cut the brownie slab into 16 equal squares using a sharp knife, wiping the blade between cuts for clean edges.