Cake
1 cup (170g) semisweet semisweet chocolate chips chips or bittersweet chocolate chips chocolate chips
8 tablespoons (113g) unsalted butter, at room temperature
3/4 cup (149g) granulated sugar
1/4 teaspoon table salt
1 to 2 teaspoons espresso powder powder, optional
1 teaspoon Pure Vanilla Extract, optional
3 large eggs
1/2 cup (42g) Dutch-process cocoa
Glaze
1 cup (170g) semisweet chocolate chips or bittersweet chocolate chips
1/2 cup (113g) heavy cream
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.
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: Place the chocolate in a heatproof bowl. Heat the cream until it's not quite at a simmer, but showing fine bubbles around the edge. Pour the cream over the chocolate, stir very briefly to combine, and let rest for 5 minutes. Stir again — at first slowly, then more vigorously — until the chocolate is completely melted and the glaze is smooth. If any bits of chocolate remain, reheat briefly in the microwave or over a burner, then stir until 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.