1. Preheat oven to 350°F.
2. Melt chocolate and butter in a double boiler.
3. Remove from heat. Add sugar, eggs, and salt.
4. Add flour, lemon zest, and 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 oven and let cool to room temperature. Refrigerate for 1 hour.
8. Remove from refrigerator, unmold, cut into individual portions, and serve.