Preheat oven to 350. Grease the bottom and sides of a 9-inch baking pan with butter. Cut a piece of parchment paper to fit the pan and place it inside. Butter the paper as well. Add 1 tsp cocoa powder to the pan and swirl to coat the inside.
Place a few inches of water in a saucepan and cover with a double boiler. Bring the water to a simmer. Add the butter and chocolate to the pan and heat until melted, stirring frequently. Remove from heat and allow to cool.
Scrape the chocolate-butter mixture into a large mixing bowl.
Whisk the egg yolks, cocoa powder, espresso powder, if using, salt, cinnamon, and cayenne pepper into the cooled chocolate mixture.
Meanwhile, place the egg whites in the bowl of stand mixer and beat on the highest speed until foamy. Gradually add the sugar, one tablespoon at a time, while the mixer is running. Continue to beat until the egg whites are glossy and hold stiff peaks.
Add a quarter of the beaten egg whites to the mixing bowl and stir to combine.
Then, add the remaining egg whites to the bowl and gently fold them into the batter until fully incorporated.
Pour the batter into the prepared pan and smooth the top.
Bake for 30-35 minutes until the center is set. A thin crust will have formed at the top. Cool on a wire rack.
For best results, allow the cake to cool completely before removing from the pan. (The cake will fall as it cools.) You can even chill the cake in the refrigerator before removing.
Garnish with whipped cream and/or fresh berries for serving.