1/2tablespoonbuttermelted, for the pan and parchment liner
Place oven rack in the middle position. Preheat oven to 375 degrees Fahrenheit.
Prepare the ingredients.
Brush the insides of a 9-inch springform pan with butter. Include the bottom. Line with parchment paper cut to fit the bottom of the pan. Brush the parchment paper with butter also.
Add an inch of water to a saucepan and bring to a simmer. Set a double boiler, glass, or metal bowl over the saucepan. Melt the chocolate and butter in the double boiler or bowl. Stir often until smooth. Remove from heat.
Add the vanilla, sugar, salt, and espresso powder to the chocolate mixture. Stir until combined.
Add a large spoonful of the chocolate mixture to the eggs to temper them. Lightly whisk to combine. Tempering ensures the eggs won’t scramble when added to the hot chocolate mixture.
Add the tempered eggs to the chocolate mixture. Whisk to combine.
Add cocoa powder to the chocolate and egg mixture. Whisk until just combined.
Pour the batter into the pan. Bake for 25 minutes or until a toothpick inserted into the center of the cake comes out clean. The top of the cake will also begin to pull away from the sides of the pan and a thin crust will form.