Using just the right ingredients—and a special shaping technique—keeps these cookies crisp.
Adjust oven rack to middle position and heat oven to 350 degrees. Line rimmed baking sheet with parchment paper.
In medium bowl, whisk together flour, salt, and baking soda.
In large bowl, whisk melted butter, sugar, and brown sugar until mixture is very well combined and smooth, about 1 minute. Add egg yolk, milk, and vanilla and whisk until well combined and lightened in color, about 30 seconds.
Add flour mixture to melted butter mixture and use rubber spatula to stir until combined and no dry flour is visible. Add chocolate chips and stir until evenly distributed.
Spray 1-tablespoon measuring spoon with vegetable oil spray. Use greased measuring spoon to drop dough onto parchment-lined baking sheet in 12 mounds (1 tablespoon each). Respray measuring spoon after every 2 or 3 mounds. Leave space between mounds.
To make cookies that are thin and crispy, we picked our ingredients carefully: cake flour, melted butter, egg yolk, and a combination of brown and white sugar. Cake flour contains less protein than all-purpose flour does, so it forms less gluten when it’s mixed into a dough (see the Taking Dough Down the Home Stretch activity). Since gluten gives baked goods their structure, a cookie with less gluten will spread out more in the oven. Using melted butter makes for a softer, moister dough that spreads out as it bakes. Like gluten, egg proteins help give baked goods structure. Skipping the protein-packed egg white keeps our cookies thin and crispy. The brown sugar adds molasses-y flavor while the white sugar holds on to less moisture to help with crispness. Last but not least: Pressing these cookies before they go into the oven encourages them to spread out evenly for optimal crispiness!