Preheat the oven to 375∞F. Lightly grease a metal 8in round cake pan.

Cut a piece of parchment to fit, grease it, and lay it in the bottom of the pan.

Put the chocolate and butter in a microwave-safe bowl and heat until the butter is melted and the chips are soft. Stir until the chips melt, reheating briefly if necessary. You can also do this over a burner set at very low heat. Transfer the melted chocolate/butter to a mixing bowl.

Stir in the sugar, salt, espresso powder and vanilla. Espresso enhances chocolate's flavor much as vanilla does; using 1 teaspoon will simply enhance the flavor, while 2 teaspoons will lend a hint of mocha to the cake.

Add the eggs, beating briefly until smooth. Add the cocoa powder, and mix just to combine.

Spoon the batter into the prepared pan.

Bake the cake for 25 minutes until the top has formed a thin crust, and it registers at least 200∞F on an instant-read thermometer when inserted into its center.

Remove the cake from the oven and cool it in the pan for 5 minutes.

Loosen the edges of the pan with a butter knife or rubber spreader and turn it out onto a serving plate. The top will now be on the bottom and the edges will crumble a bit. Allow the cake to cool completely before glazing.