Preheat oven to 350°F. Grease a 9 inch tart pan or 9 inch spring-form pan, line with a parchment paper round, then grease the parchment paper.
In a medium bowl, heat the chocolate and butter in the microwave in 30 second intervals until the chocolate is melted and smooth. Let it cool for 5 minutes.
Whisk in the egg yolks, sugar, cocoa powder, espresso powder, vanilla extract and salt until everything is combined.
In a large bowl, use an electric mixer and whip the egg whites on medium speed until you have stiff peaks. Gently fold the egg white mixture into the chocolate mixture until just combined. Transfer to the prepared pan and bake for 25-30 minutes until the center is no longer jiggly. Let the cake cool for 10 minutes before removing from pan.
While cake is cooling, make the whipped cream. Mix the espresso powder with warm water in a small bowl and let cool. Use a hand mixer or stand mixer with whisk attachment and whisk heavy cream, sugar, cocoa powder, espresso and vanilla extract on medium-high speed until medium peaks form, about 3-4 minutes. Use when ready.
Serve warm, room temperature or cold. Top with some cocoa power, powdered sugar, raspberries or whipped cream.