1. Preheat oven to 350°F (175°C) and position a rack in the center

2. Cream together softened butter, brown sugar, and granulated sugar until pale and fluffy

3. Stir in egg, sourdough starter, and vanilla extract until fully combined

4. In a separate bowl whisk flour, baking soda, and sea salt then fold into wet ingredients until just mixed

5. Press dough evenly into a 9-inch tart pan with removable bottom and dock with a fork

6. Bake tart shell for 12 to 15 minutes until edges are golden and center is set then cool completely

7. Heat heavy cream until steaming and pour over chopped dark chocolate, let sit one minute and stir into a smooth ganache

8. Spread ganache over cooled tart shell in an even layer and let set for 10 minutes

9. Sprinkle sea salt flakes evenly over ganache and add optional toasted nuts if desired

10. Slice and serve at room temperature with coffee or milk