Position racks in upper and lower thirds of oven. Preheat to 350°F. Line two large baking sheets with parchment paper.

In a medium bowl, whisk together flour, espresso powder, baking soda, and salt until the espresso is evenly distributed and no dark streaks remain.

In a large bowl with a mixer, beat butter, brown sugar, and granulated sugar on medium-high speed until light and fluffy, about 3 minutes. Scrape down bowl once or twice.

Beat in whole egg, egg yolk, and vanilla until fully incorporated, about 1 minute. The mixture should look smooth and slightly lighter in color.

With mixer on low, gradually add flour mixture and beat just until no dry spots remain. Do not overmix.

Using a wooden spoon or rubber spatula, fold in chocolate chunks until evenly distributed. The dough will be thick and slightly sticky.

Scoop dough into 2-tablespoon balls (about 1.5 inches) and place 2 inches apart on prepared sheets. If dough is very soft, chill 10 minutes before baking.

Bake until edges are set and golden brown but centers still look slightly underdone, 11 to 13 minutes, rotating sheets halfway through. They will firm up as they cool.

Let cookies cool on sheets 5 minutes, then transfer to wire racks to cool completely. The espresso flavor intensifies as they rest.