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.
Set a heatproof bowl over a saucepan of simmering water. Put the chocolates and butter in the bowl and stir occasionally until the ingredients are just melted - you don't want them to get so hot that the butter separates. Remove the bowl from the pan of water.
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 pan and smooth the top with the spatula.
Bake the brownies 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 onto a cutting board. Cut into 16 squares, each a scant 2-inches on a side. Serve at either room temperature or slightly chilled. Yield: 16 brownies