Preheat oven to 350F. Place rack in middle position.
Prepare 8-inch round cake pan by spraying with cooking spray, layering with a parchment round, then spraying again.
In a large bowl, place butter and chopped chocolate.
Create a double boiler by pouring one inch of water in medium saucepan and placing over medium heat, then place bowl on rim of the saucepan. (The bottom of the bowl should not touch the simmering water below.) Stir occasionally until most of the chocolate is melted. Remove from heat and continue stirring until all the chocolate is melted.
Stir in sugar, vanilla, and dissolved espresso. (Mixture will look grainy.)
Whisk in eggs until mixture is thick and smooth, about one minute.
Add salt and sift cocoa powder into the batter and evenly combined. Add batter to prepared pan.
Bake until top is just set and an inserted toothpick comes out with a few very moist crumbs attached, 16-18 minutes. DO NOT OVERBAKE.
Let cool in pan for 5 minutes. Invert cake onto serving platter and allow to cool for 15 minutes. Garnish with whipped cream and berries and serve immediately.