Preheat the oven to 350°F (180°C). Grease an 8-inch round cake pan with nonstick cooking spray and line the bottom with parchment paper.
In a large microwave safe bowl, add the chopped chocolate and cubed butter. Microwave in 20 to 30-second intervals, stirring in between each increment, until the chocolate is completely melted and the mixture is smooth. Allow to cool for 5 minutes before proceeding with the next step.
Add the granulated sugar and vanilla to the cooled chocolate mixture and whisk until well combined, then whisk in the eggs.
Add the cocoa powder, espresso powder, baking powder, and salt into the bowl and mix until just combined. Don’t overmix the batter.
Transfer the cake batter to the greased pan and use a knife or spoon to spread the batter into an even layer.
Place the pan on the center rack of the oven and bake for 28 to 33 minutes or until a toothpick inserted into the center comes out mostly clean with just a few moist crumbs.
Remove from the oven and cool in the pan for 5 to 10 minutes. Carefully invert the cake onto a cooling rack, immediately flip right side up onto another cooling rack, and allow to cool completely.
Top the cake with confectioners sugar, whipped cream or chocolate ganache! Alternatively, serve with a scoop of vanilla ice cream.