Preheat oven to 350°F (177°C). Grease an 8-inch round cake pan, line with parchment paper rounds, then grease the parchment paper.

Cut the butter into pieces and place in a large heat-proof bowl. Add chopped chocolate and melt in 20 second increments in the microwave, stirring after each increment until completely melted and smooth. Let cool for 2-3 minutes.

Whisk the sugar, espresso powder, and vanilla extract into the chocolate mixture. Whisk in the eggs until smooth. Whisk in the cocoa powder, salt, and baking powder (if using).

Pour and spread batter into prepared cake pan.

Place a large metal roasting pan or baking dish on the bottom oven rack. Pour boiling water about 2 inches up the sides of the pan. Quickly place the cake on the center rack and shut the oven door.

Bake for 30 minutes until the edges are set. Begin checking it at 25 minutes.

Remove from the oven, place cake on a cooling rack, and cool for only 10 minutes in the cake pan. Run a sharp knife around the edges to help release the warm cake, then quickly invert it onto a serving plate or cake stand. Cool completely.

Once the cake is cool, top with your choice of toppings.

For Mocha Whipped Cream: Mix the espresso powder and warm water together. Whip the heavy cream, sugar, cocoa powder, and espresso mixture on medium-high speed until medium peaks form.

Serve cake with whipped cream or pipe it on top. Decorate with raspberries and a dusting of confectioners’ sugar, if desired.