Preheat oven to 325 F. Line the bottom of an 8" cake pan with parchment paper, and set aside.
Pull out a baking sheet while you're at it, and boil water in a kettle–you're gonna use them later.
Prepare your double boiler by placing a large heatproof bowl over a pot of simmering water. The bowl should be slightly wider than the pot so that it snugly fits the pot without touching the water.
Melt the semisweet chocolate chips and butter in the bowl of the double boiler, stirring occasionally so that they incorporate.
Remove the bowl from heat. Whisk in 5 egg yolks until the chocolate is smooth and silky.
In a stand mixer, beat 5 egg whites, vanilla, cinnamon, and cream of tartar until stiff peaks form. Fold these whipped egg whites in with the chocolate until smooth.
Pour the chocolate batter into the cake pan and place the cake pan on the baking sheet. Carefully fill the baking sheet half way with hot water, and carefully transfer the sheet to the oven.
Bake for 30 minutes.
Carefully remove the cake from the oven, and cool completely on a wire rack. Transfer to the fridge to chill the cake overnight.
The next day, invert the cake onto a serving plate and carefully unmold it from the cake pan. Peel off the parchment paper, and top with the ganache.
Serve the cake with whipped cream and berries.