Line a square 8x8 baking pan with parchment paper on all sides and preheat the oven to 350 F/180 C.
In a medium bowl, whisk together the sugars, egg, egg yolk, vanilla, and ground espresso until light and pale in color, about 1 minute.
⅔ cup light brown sugar, ¼ cup granulated sugar, 1 large egg, 1 large egg yolk, 1 teaspoon vanilla extract, 1 teaspoon ground espresso or coffee
Slowly stream the hot melted butter into the bowl, whisking constantly to combine.
½ cup unsalted butter
Fold in the flour, baking powder, salt, and chocolate chips until just combined.
1 cup + 2 tablespoon all-purpose flour, ⅔ cup semi-sweet chocolate chips, ¼ teaspoon baking powder, ½ teaspoon salt
Spread the batter into the prepared baking pan.
Bake for 15-20 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs.
Remove the pan from the oven and press a few more chocolate chips on top if desired.
Let the pan cool completely on a wire cooling rack before cutting, then enjoy!