Grease an 8" (I used 9" but I prefer 8" for a thicker cake) springform cake pan. Cut a parchment circle to fit in the bottom of the pan. Grease the parchment as well. Dust with 1 tablespoon cocoa powder and tap out excess. Preheat oven to 375 degrees.
Place butter and chocolate in a medium saucepan (or you could microwave carefully). Heat on low until butter is melted and chocolate has softened. Remove and stir to melt chocolate and smooth mixture.
Add sugar, salt and vanilla. Whisk well to combine. Add eggs, one at a time and whisk well after each addition. Stir in the cocoa powder and whisk to combine. Pour batter into pan. Bake 25 minutes. Remove from the oven and let cool 10 minutes. Run a knife around the edge of the pan. Invert cake onto a serving platter. Top with ganache if desired, and/or berries and whipped cream.
Ganache: Heat the cream and corn syrup to scalding. Pour over the chips in a heat proof bowl. Let sit for 1 minute to melt chocolate. Stir gently to smooth. Stir in vanilla and butter. Stir until butter melts and mixture is again smooth.
