Step 1

Preheat the oven to 350°F and set an oven rack in the middle position. Grease a 9-inch springform pan with nonstick cooking spray.

Step 2

In a large microwave-safe bowl, melt the butter in the microwave until hot and foamy (careful not to boil it, or it will splatter all over your microwave). Add the chocolate and espresso powder (if using), and whisk until the chocolate is completely melted and smooth. (If necessary, pop the bowl back into the microwave for 20 seconds to melt any remaining bits of chocolate.)

Step 3

Add the granulated sugar to the melted chocolate mixture. Using an electric mixer, beat on low speed until well combined, about 30 seconds. Add the eggs one at a time, beating on low speed between each addition for about 20 seconds. Quickly beat in the vanilla. Add the cocoa powder and mix on low speed until evenly incorporated. Pour the batter into the prepared pan and bake for 32 to 35 minutes, or until the cake is puffed and set (it will likely crack a bit on top; that's normal and part of the charm of the cake). It's a little tricky to test for doneness, but if you insert a toothpick or cake tester into the center of the cake through one of the cracks, you should get wet crumbs. Let the cake cool on a rack for 15 minutes, then remove the ring on the springform pan and allow the cake to cool completely.

Step 4

No more than 3 hours before serving, make the topping. In the bowl of an electric mixer fitted with the whisk attachment (or beaters), whip the heavy cream until slightly thickened. Sprinkle the confectioners' sugar over the cream and beat until soft, pillowy peaks form. Do not overbeat.

Step 5

Dollop and swirl the whipped cream decoratively over the cake. Sift a bit of cocoa powder over the cream and decorate with chocolate curls (if using).

Step 6

Make-Ahead Instructions: The base of the cake can be made up to one day ahead of time and stored in a covered container in the refrigerator. Allow the cake to come to room temperature before topping with whipped cream and serving. Leftover cake can be stored in a covered container in the refrigerator for up to 3 days (bring to room temperature before serving).