1. Preheat the oven to 350°F.
2. Melt the chocolate and butter in a double boiler.
3. Remove from heat. Add sugar, eggs, extract, and salt.
4. Add flour and zest; mix.
5. Pour into the mold and bake for 15 minutes.
6. Let rest for 5 minutes in the turned-off oven.
7. Remove from the oven and let cool to room temperature. Refrigerate for 1 hour.
8. Remove from the refrigerator, unmold, cut into individual portions, and serve.