1. Brown the butter in a light-colored saucepan over medium heat, swirling until golden brown and nutty, about 5-7 minutes; transfer to a bowl and cool 10-15 minutes.

2. Whisk cooled brown butter with brown sugar and granulated sugar until combined and glossy.

3. Beat in the egg, egg yolk, and vanilla extract until smooth.

4. Stir in sourdough starter until fully incorporated.

5. In a separate bowl, whisk flour, baking soda, and salt, then fold into the wet mixture until no streaks of flour remain.

6. Fold in chocolate chips and dried cranberries evenly.

7. Cover and chill the dough for at least 2 hours or overnight.

8. Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

9. Scoop dough into 1½-tablespoon portions, spacing 2 inches apart on the prepared sheets.

10. Bake for 10-12 minutes until edges are set and centers are slightly underbaked; sprinkle with flaky sea salt if desired.

11. Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.