Preheat the oven to 350° F. Line a 9-inch square cake pan with foil, draping the foil over the edges. Lightly butter the foil, and set aside.
In a large saucepan, melt the butter with the unsweetened chocolate over very low heat, stirring occasionally. Remove from the heat. Whisking them in one at a time until thoroughly incorporated, add the cocoa, sugar, eggs, vanilla, flour, salt, and espresso powder. Pour the batter into the prepared pan, and smooth the surface.
Bake the fudge brownies in the center of the oven for about 30-35 minutes, until the edge is set but the center is still a bit soft and a toothpick inserted into the center comes out coated with a little of the batter. Let the brownies cool at room temperature in the pan for 1 hour, then refrigerate just until they are firm, about 1 hour. Lift the brownies from the pan and peel off the foil. Cut the brownies into 16 squares. Serve at room temperature.
Notes
*Use 2 eggs for a super rich dense fudgey brownie. Use 3 eggs for a little more cakey brownie Make Ahead: The Salted Fudge Brownies can be refrigerated for up to 3 days, and frozen for up to 1 month.Adapted from Food and Wine