1. Preheat oven to 350°F (175°C) and prepare a tart pan with removable bottom.

2. In a heatproof bowl, melt dark chocolate and butter together until smooth, using a double boiler or microwave bursts.

3. Whisk granulated sugar and brown sugar into the melted chocolate mixture until fully combined.

4. Stir in sourdough starter until the mixture is smooth and slightly thickened.

5. Add the egg and vanilla extract, mixing gently until glossy and cohesive without overmixing.

6. In a separate bowl, whisk flour and cocoa powder, then fold into the wet mixture until a pliable dough forms.

7. Rest the dough for at least 30 minutes, then press evenly into the prepared tart pan.

8. Bake for 20–25 minutes until the edges are set but the center still wobbles slightly.

9. Immediately sprinkle flaky sea salt over the warm tart, then cool in the pan for 30 minutes before unmolding.

10. Serve warm or at room temperature with whipped cream, ice cream, or espresso, and store in an airtight container for up to five days.