Mix Butter and Sugars: In a large bowl, cream together softened butter, brown sugar, and granulated sugar until smooth.
Add Eggs and Vanilla: Beat in the egg, egg yolk, and vanilla extract until fully incorporated.
Combine Dry Ingredients: In a medium bowl, whisk together flour, espresso powder, baking soda, and salt. Gradually mix into the wet ingredients until just combined. A few lumps are okay.
Fold in Chocolate: Gently fold in chocolate chunks or chips by hand. Do not overmix.
Chill Dough: Refrigerate dough for at least 30 minutes, or up to 2 hours. Chilling helps the cookies hold their shape.
Scoop Cookies and Bake: Use a 1 oz cookie scoop to portion dough balls, spacing 2 inches apart on prepared sheets and bake them in oven at 350°F (175°C) for 10–12 minutes until edges are lightly golden and centers are soft.
Cool and Garnish: Let cookies cool 5 minutes on the baking sheet. Sprinkle with flaky sea salt if desired. Serve warm or at room temperature.