In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, cream the softened butter, brown sugar, and white sugar on medium speed. Mix for 2-3 minutes until the mixture is light in color and has a fluffy, paste-like consistency. Avoid over-creaming, as this can incorporate too much air and cause the cookies to deflate after baking.
In a separate medium bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt. Whisking ensures the leavening agents are evenly distributed, which is essential for a consistent rise and texture in every cookie.
Reduce the mixer speed to low and add the cold egg and vanilla extract to the butter mixture. Mix until just combined, scraping down the sides of the bowl with a spatula as needed to ensure everything is incorporated.
With the mixer on its lowest speed, gradually add the dry ingredient mixture to the wet ingredients in 3-4 additions. Mix only until the last streaks of flour have disappeared. Do not overmix. Overmixing develops gluten, which will result in tough, dense cookies rather than soft and chewy ones.
Remove the bowl from the mixer and fold in the milk chocolate chips using a spatula. Reserve a handful to press onto the tops of the cookies before baking for a picture-perfect look.
This is the secret to a true Crumbl-sized cookie. Use a kitchen scale to portion the dough into 4.5 to 5-ounce balls (about the size of a tennis ball). For that signature craggy top, roll the dough into a ball, then gently break it in half and press the two halves back together with the jagged, broken edges facing up. This creates more surface area for a deliciously textured top.
Place 4-5 dough balls on each prepared baking sheet, ensuring they are spaced at least 3 inches apart to allow for spreading. Bake for 10-12 minutes. The edges should be set and lightly golden brown, but the centers should still look slightly soft and underdone. This is the key to a gooey center.
Remove the baking sheets from the oven and let the cookies cool on the pan for at least 10 minutes. This is a crucial step, as the cookies will continue to bake from the residual heat of the pan, allowing the center to set up perfectly. After 10 minutes, carefully transfer the cookies to a wire rack to cool completely.