• Preheat the oven to 360 and line a 9 inch square pan with parchment paper. Leave an overhang of parchment so that you can easily lift the brownies from the pan once they’re cool. 
• Melt the butter, and whisk in the brown sugar and granulated sugar.
• Add the eggs and vanilla and whisk for 3-4 minutes, until the mixture is glossy and lightened in color.
• Fold in the cocoa powder, flour, baking powder, and salt until just combined.
• Fold in the chopped 40% chocolate.
• Pour the brownie batter into a prepared pan and use an offset spatula to spread into an even layer.
• Bake for 30 minutes at 350. Remove from the oven and let them cool completely.

For the frosting –

• Melt the chocolate and butter together until completely smooth. Let cool for 5-10 minutes.
• Whisk in the espresso powder and powdered sugar. The frosting will seem thin, but will thicken up as it comes to room temperature.
• Once frosting is thick and spreadable, spread into an even layer on the cooled brownies. Top with flaky salt and slice into squares.