Preheat Oven & Prep Sheets:
Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

Melt Chocolate & Butter:
In a medium saucepan over low heat, melt butter with 2½ cups chocolate chips until smooth. Let cool 10–15 min.

Beat Eggs & Sugars:
In a large bowl, whisk eggs, egg yolks, white sugar, brown sugar, and vanilla on medium-high speed 3–4 min until thick and glossy.

Combine Wet Ingredients:
Stir in sourdough discard, then slowly mix in cooled chocolate-butter mixture.

Add Dry Ingredients:
Sift together flour, cocoa, salt, and optional espresso powder. Mix into wet batter on low speed just until combined. Do not overmix.

Fold in Chocolate Chips:
Gently fold remaining ½ cup chocolate chips (or chunks) into batter.

Chill Dough (Optional but Recommended):
Cover and chill 30–60 min for thicker, chewier cookies.

Scoop & Space:
Using a cookie scoop or 2 tablespoons, drop dough 2 inches apart on prepared sheets.

Bake:
10–12 min until edges look set but centers still appear soft and slightly puffed.

Finish & Cool:
Immediately sprinkle with flaky sea salt and extra chocolate chunks. Cool 5 min on the sheet, then transfer to wire rack.