Preheat the oven to 350ºF (177ºC). Prepare an 8-inch round pan by spraying the sides with baking spray and lining the bottom with parchment paper, then spray the parchment paper as well.
In a large microwave safe mixing bowl, add the chocolate chips (or chopped chocolate). Slice the butter into tablespoon-sized pieces so it melts easily and add it in with the chocolate. Microwave in 30-second intervals, stirring after each one, until the butter has melted and the mixture can be whisked into a completely smooth, uniform consistency. Let the chocolate mixture cool for 2-3 minutes.
To the chocolate mixture, add the sugar, espresso powder (or instant coffee) if using, vanilla, and salt. Whisk together until combined, then whisk in the eggs and egg yolk until smooth. Add the cocoa powder and whisk until combined. The batter will be thick like brownie batter.
Add the heavy whipping cream into a separate medium bowl and use a hand mixer to whip it into soft peaks, about 2 minutes. You can alternatively use a stand mixer with the whisk attachment for this. Once the whipped cream has reached soft peaks, use a rubber spatula to gently fold it into the chocolate mixture until no streaks remain.
Pour the batter into the prepared cake pan and bake for 25-30 minutes, until the edges are set and an instant read thermometer inserted into the center reaches 200ºF (93ºC). If you don't have an instant read thermometer, check for doneness by inserting a toothpick into the center and seeing that it comes out clean. It's better to slightly underbake this cake than overbake it, so start checking for doneness at 25 minutes. It's normal for the sides of the cake to rise higher than the center and to see some minor cracks in the surface.
Let the cake cool in the pan for no more than 5-10 minutes, then run a sharp knife around the edges to release them from the sides of the pan. Then, invert it onto a serving plate so that the bottom of the cake becomes the top. Let the cake cool completely by placing it in the refrigerator for 1-2 hours.