1. Brown the butter in a saucepan over medium heat until amber brown and nutty, then pour into a bowl and let cool to lukewarm

2. Whisk together flour, baking soda, and salt in a medium bowl

3. In a large bowl, beat cooled brown butter with brown sugar and granulated sugar until thick and glossy

4. Add egg, egg yolk, and vanilla extract and mix until smooth

5. Fold in sourdough discard until fully incorporated

6. Gently fold in dry ingredients until no white streaks remain

7. Stir in chocolate chunks

8. Cover dough and refrigerate for at least 30 minutes

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

10. Roll chilled dough to 1/4 inch thickness on a lightly floured surface and cut with a heart-shaped cutter

11. Place hearts on prepared sheets, chill for 15 minutes, then bake 9–11 minutes until edges set and centers are soft

12. Remove from oven, sprinkle with sea salt flakes if desired, let cool on sheet for 2 minutes, then transfer to a rack to cool completely