Grease a 13x9 inch baking pan, and line with aluminum foil or buttered parchment paper.
In a small bowl mix the espresso powder with the vanilla extract until powder dissolves.
In the top part of a double boiler set over barely a simmering water, melt the butter and the chocolate pieces together. Remove from heat, and let cool for 15 minutes.
In a large bowl, beat the eggs, and whisk in the salt, sugars and vanilla mixture. Fold in the flour until just combined. Stir in the morsels. Pour the batter in the prepared pan.
Bake for 35 to 40 minutes or until the surface looks shiny and begins to crack. To test doneness, insert a toothpick into the center. It should come out with a few moist crumbs. Let the brownie cool in the pan on a rack before cutting and serving.