Preheat the oven to 375°F. Lightly grease a metal 8″ round cake pan; cut a piece of parchment to fit, grease it, and lay it in the bottom of the pan. See “tips,” below.
To make the cake: 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; the top will have formed a thin crust, and it should register at least 200°F on an instant-read thermometer inserted into its center.
Remove it from the oven, and cool it in the pan for 5 minutes.
Loosen the edges of the pan with a table knife or nylon spreader, and turn it out onto a serving plate. The top will now be on the bottom; that’s fine. Also, the edges will crumble a bit, which is also fine. Allow the cake to cool completely before glazing.
To make the glaze: Combine the chocolate and cream in a microwave-safe bowl, and heat until the cream is very hot, but not simmering. Remove from the microwave, and stir until the chocolate melts and the mixture is completely smooth.
Spoon the glaze over the cake, spreading it to drip over the sides a bit. Allow the glaze to set for several hours before serving the cake.