Prep the pan: Preheat oven to 350°F. Grease an 8- or 9-inch springform pan and line the bottom with parchment paper.
Melt chocolate and butter: Add chopped chocolate and butter to a heat-safe bowl. Melt gently until smooth and glossy.
Whisk eggs and sugar: In a separate bowl, whisk eggs and sugar until smooth and slightly lighter in color.
Combine mixtures: Slowly whisk the melted chocolate mixture into the eggs until fully blended.
Add cocoa and flavorings: Sift in cocoa powder, then add vanilla and salt. Fold until the batter is thick and smooth.
Bake: Pour batter into the pan and smooth the top. Bake for 25–35 minutes, until edges are set and the center still has a slight wobble.
Cool and slice: Cool in the pan for 15–20 minutes, loosen the edges, then cool completely before slicing.
