1-Gather and prep: Line two baking sheets with parchment paper and measure all ingredients.

2-Melt chocolate mixture: Melt butter with 1 cup chocolate chips over medium heat, stirring, until smooth (2‑3 min).

3-Add cocoa and espresso: Remove from heat; stir in cocoa powder and optional espresso powder until incorporated.

4-Beat wet ingredients: Beat eggs, extra yolk, sugar, oil, and vanilla until pale and frothy, about 2 minutes.

5-Combine with chocolate: Pour the warm chocolate mixture into the egg mixture and beat until fully combined.

6-Fold in dry ingredients: Fold in flour, baking powder, and salt just until no streaks remain; do not overmix.

7-Scoop the dough: Using a 1½-tablespoon scoop, portion dough balls onto sheets, spacing 2 inches apart.

8-Sprinkle with sea salt: Sprinkle a few flakes of flaky sea salt on each dough ball.

9-Bake the cookies: Bake just above center at 350 °F (177 °C) for 8‑10 minutes, until tops are shiny and crackly but centers are soft.

10-Add chocolate chips: Immediately press 3‑4 extra chocolate chips into the top of each warm cookie.

11-Cool and set: Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.