Preheat oven to 325°F. Line the bottom of an 8-inch square baking pan with parchment paper; butter the parchment paper and pan sides with butter. Set aside.
Put the chocolates and butter in the top of a double boiler set over simmering water. Stir occasionally until the ingredients are just melted - you don't want them to get so hot that the butter separates. Remove the top of the double boiler.
With a whisk, stir in the sugar. Don't be concerned when your smooth mixture turns grainy. One by one, whisk in the eggs. Add the vanilla and give the ingredients a vigorous whisking before gently stirring in the espresso, salt, and flour; stir only until incorporated. Switch to a rubber spatula and fold in the chopped pecans.
Scrape the batter into the prepared pan and smooth the top with the spatula.
Bake the brownies in a preheated 325°F oven for 30 to 33 minutes, or until the top is dull and a thin knife inserted into the center comes out clean. Transfer the pan to a rack and cool the brownies to room temperature.
When the brownies are completely cool, turn out onto a rack, peel away the parchment paper, and invert them onto a cutting board. Cut into 16 squares, each about 2 inches. Serve at either room temperature or slightly chilled. Yield: 16 brownies.