4 oz bittersweet chocolate (good-quality, not unsweetened)
1 stick (1/2 cup) unsalted butter
1/2 tsp bourbon vanilla extract
3/4 cup white sugar
3 large eggs
1/2 tsp ground cinnamon
1/2 cup unsweetened cocoa powder
Preheat oven to 375°F. Butter an 8-inch round baking pan. Line the bottom with parchment paper and butter the paper. Chop chocolate into small pieces. In a metal bowl, set over a saucepan of just simmering water, melt the chocolate and butter, stirring until smooth. Add in vanilla and stir. Remove bowl from heat and whisk sugar into chocolate until combined. Add eggs and whisk until well incorporated. Add cinnamon. Sift cocoa powder over chocolate mixture and whisk until combined. Pour batter into greased pan and bake in middle oven for ~ 25 minutes. Cool cake in pan on a cooling rack for 5-7 minutes and then invert onto a serving plate.