Heat the oven to 350°F and arrange a rack in the middle. Line an 8-by-8-inch glass baking dish with aluminum foil.
Combine the chocolate and butter in a medium saucepan and cook over medium-low heat, stirring frequently, until evenly melted. Remove from the heat and let cool to room temperature.
Combine the eggs, sugar, cocoa powder, vanilla, espresso (if using), and salt in a large bowl and briefly stir until just evenly incorporated. Add the cooled chocolate and mix until uniform in color. Add the flour and stir until just incorporated (no white streaks should remain).
Transfer the batter to the prepared baking dish and bake until a tester inserted into the center of the brownies comes out clean, about 25 to 30 minutes. Remove from the oven and let cool completely before cutting.
Beverage pairing:Rogue Chocolate Stout, Oregon. Few things are more delicious than the combination of bitter and sweet. This dark beer brewed with bittersweet chocolate brings a toasty, dark palate to the table, as well as the slightly bitter flavors of deeply roasted malt. And its acidity and creamy carbonation provide a mouth-cleansing rinse.