Place a rack in the center of the oven. Heat the oven to 450 °F. Line a round 6-inch baking pan with 3-inch sides completely with parchment paper making sure to press the paper completely into the edges of the pan and leave a few inches of paper to hang over the sides.
Place the chocolate in a small bowl and melt in the microwave using 20-30 second intervals, stirring in between until the chocolate is fully melted. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment beat the whipped cream until light and fluffy. Beat in the sugar and continue to mix until well combined. Sift in the cocoa powder and espresso and continue to beat until smooth making sure that no lumps remain.
Beat in the eggs, one at a time, and the vanilla. Using a rubber spatula gently fold in the melted chocolate. Add the cream in two additions beating to combine.
Pour the batter into the prepared baking pan and bake for 25-28 minutes or until the cake is burnt across the top, puffed up around the edges and still slightly loose in the center. Turn off the oven, prop the door open with a wooden spoon and allow the cake to rest in the oven for 15 minutes.
Remove the cake from the oven and transfer it immediately to the refrigerator. Allow the cake to chill for a minimum of 4 hours or overnight. Serve at room temperature topped with whipped cream and berries, if using.