Lightly butter the bottom and sides of a 10-inch springform pan. Dust the bottom and sides with the cocoa, shaking to remove excess.
Combine the bittersweet chocolate and butter in a large microwave-safe bowl and microwave in 30-second intervals, stirring in between each, until completely melted and smooth, about 1 minute. (Alternatively, melt in a small metal bowl set over a saucepan of simmering water, stirring until smooth). Let cool slightly. Whisk in the granulated sugar. Whisk in the eggs 1 at a time, whisking well after each addition. Sift the remaining 1 cup cocoa over the mixture and whisk until just combined.
Pour the batter into the prepared pan and bake until just barely set, around 30 minutes. Transfer to a wire rack; let cool for 10 minutes. Run a knife around the edges to loosen and remove sides. Remove to a serving platter and let cool completely. Dust with powdered sugar. Serve with cinnamon ice cream.