Preheat your oven to 350°F and line a baking tray with parchment paper. While the oven heats, gather all your ingredients and measure them out—this ensures you won't be scrambling mid-mixing. I like to have everything prepped because these cookies come together so fast that there's no time to hunt for ingredients once you start.

In a small bowl, combine the just-melted butter with the packed brown sugar, stirring until well combined and the sugar begins to dissolve slightly into the warm butter. Add the room-temperature egg yolk and vanilla extract, whisking gently until the mixture is smooth and homogeneous. This warm butter will help emulsify the egg and create a cohesive dough base.

In a separate small bowl, whisk together the flour, baking soda, baking powder, salt, and optional espresso powder. Pour the wet ingredient mixture from Step 2 into the dry mixture and fold together with a spatula until just combined—don't overmix, as this will develop gluten and make the cookies tough. The dough should come together quickly but still look slightly shaggy.

Gently fold the chocolate chips into the dough using a spatula until evenly distributed. Using a small spoon or cookie scoop, portion the dough onto your prepared baking tray, spacing them about 2 inches apart—these cookies will spread slightly as they bake. I find that slightly underfilled scoops actually work better here; they crisp up at the edges while staying chewy in the center, which is exactly what we want in a 20-minute cookie.

Bake for 8-10 minutes, until the edges are set but the centers still look slightly underdone—they should appear just barely set when you pull them from the oven. The cookies will continue to cook on the hot tray after removal, so resist the urge to bake them until they look fully done or they'll be crunchy rather than chewy.

Let the cookies cool on the tray for 2-3 minutes until they're firm enough to handle but still warm and pliable. If you'd like slightly thicker, puffier cookies, gently shape them into rounded mounds while warm using the back of a spoon or a small bowl. Transfer to a cooling rack or plate to cool completely. These are best enjoyed while still slightly warm, or store them in an airtight container for up to 3 days.