Preheat the oven to 350 degrees F. Grease sides and bottom of a 9x13-inch glass or light-colored baking pan. Line the pan with parchment paper.

In a medium bowl, whisk the flour, salt, and cocoa powder together.

Combine the chocolate, butter and instant espresso powder in the top of a double boiler and melt gently, stirring often, until smooth. Turn off the heat, but keep the bowl over the water and add the sugars. Whisk until blended, then remove the bowl from the pan and let cool (if needed) to room temperature.

Whisk 3 eggs into the chocolate mixture. Add the remaining eggs and whisk until combined. Stir in the vanilla. Do not overbeat the batter at this stage or your brownies will be cakey.

Sprinkle the flour mixture over the chocolate mixture, and using a rubber spatula or scraper (not a whisk), fold the flour mixture into the chocolate until blended.

Pour the batter into the pan and smooth the top. Bake in the center of the oven for 30 minutes, rotating the pan halfway through the baking time, until a toothpick inserted into the center of the brownies comes out with a few moist crumbs sticking to it.

Let the brownies cool completely, then lift from pan. Cut into squares and serve at room temperature, or chill before cutting and bring back to room temperature before serving.

Store at room temperature in an airtight container or wrap with plastic wrap for up to 3 days or wrap tightly and freeze.

(Recipe adapted from Baked: New Frontiers in Baking)