Prep the basics. Line two baking sheets with parchment paper. Set the oven to 350°F (175°C). If your butter is hot, let it cool until just warm to the touch so it doesn’t cook the eggs.

Whisk the wet ingredients. In a large bowl, whisk the melted butter, brown sugar, and granulated sugar until smooth and glossy, about 30 seconds. Whisk in the egg, egg yolk, and vanilla until combined.

Combine the dry ingredients. In a separate bowl, whisk the flour, baking soda, salt, and cornstarch. This helps prevent clumps and keeps the dough uniform.

Bring the dough together. Add the dry ingredients to the wet and stir with a spatula just until no dry streaks remain. Fold in the chocolate chips or chunks. The dough will be soft and a bit glossy.

Chill briefly. Cover and chill the dough for 20–30 minutes. This step helps the cookies bake thicker and improves texture.

Portion the cookies. Scoop 2-tablespoon mounds (about 40–45 g each) and place them 2 inches apart on the lined sheets. If you like, press a few extra chocolate chips on top for a bakery look.

Bake. Bake one sheet at a time for 9–11 minutes until edges are set and lightly golden, but centers still look soft and slightly underdone. Don’t overbake—carryover heat will finish them.

Finish and cool. If using, sprinkle with a pinch of flaky sea salt right out of the oven. Let cookies cool on the sheet for 5–7 minutes, then move to a rack to cool completely (or enjoy warm).

Adjust for preferences. For extra softness, pull at 9 minutes. For firmer edges, go closer to 11–12 minutes, watching closely.