Get Ready to Bake: Preheat your oven to 350°F (175°C) and position an oven rack in the middle. Prepare an 8-inch round cake pan by lightly spraying it with cooking spray, lining the bottom with a parchment paper circle, and then spraying the parchment too.
Melt the Chocolate Magic: In a heatproof bowl, combine the butter pieces and chopped bittersweet chocolate. Create a simple double boiler: place the bowl over a saucepan with about an inch of simmering water (make sure the bowl doesn't touch the water). Gently stir until the chocolate and butter are mostly melted. Remove the bowl from heat and continue stirring until everything is smooth and fully melted.
Sweeten It Up: Stir the granulated sugar, vanilla extract, and dissolved espresso into the melted chocolate mixture. Don't worry if it looks a little grainy at this stage.
Whisk in the Eggs: Add the room temperature eggs to the mixture and whisk steadily for about a minute until the batter becomes wonderfully thick and smooth.
Finish the Batter: Sprinkle in the salt and sift the cocoa powder directly into the batter. Gently fold or stir until everything is perfectly combined and there are no dry streaks.
Pour and Bake: Pour your luscious chocolate batter into the prepared cake pan. Bake for 16-18 minutes, or until the top looks set and a toothpick inserted into the center comes out with just a few moist crumbs attached. Be careful not to overbake!
Cool Down: Let the cake cool in the pan for 5 minutes. Then, carefully invert it onto a serving plate and let it cool for another 15 minutes before slicing.
Serve and Enjoy: Garnish with a dollop of fresh whipped cream and a sprinkle of berries for a truly delightful treat!