Preheat oven to 350 degrees F, and line the bottom and sides of an 8-inch square baking pan with parchment paper.
In a medium bowl, whisk together flour, cocoa powder and salt.
Place the chocolate, butter and coffee in a large heat-proof bowl set over a pot of simmering water, stirring occasionally, until the mixture is completely melted and smooth.
Take bowl off heat and whisk in the granulated sugar, brown sugar and vanilla until completely combined. Add eggs and whisk quickly to combine. Sprinkle flour mixture over the chocolate batter. Using a rubber spatula, fold the flour into the chocolate until just combined.
Pour batter into the prepared tin and bake in oven 25 to 30 minutes. The center will still be slightly gooey. Let cool slightly before removing from pan - the batter will firm up as it cools.
Once cooled completely, cut into squares to serve. Store in an airtight container at room temperature for up to 3 days.