📖 Instructions

1. Before you do anything, preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving a slight overhang on the sides. This makes lifting the baked slab out super easy later on. If you don't have parchment, grease and flour the pan well.
2. In a large mixing bowl, either with a stand mixer fitted with the paddle attachment or with a hand mixer, cream together the softened unsalted butter, granulated sugar, and light brown sugar. Beat them on medium speed until the mixture is really light, fluffy, and looks pale, which usually takes about 2-3 minutes. This step is super important for that soft, chewy texture.
3. Crack in the large egg and add the vanilla extract to the creamed butter and sugar. Beat again on medium speed until everything is just combined and smooth. Be careful not to overmix here; we just want it to come together.
4. In a separate, medium-sized bowl, whisk together your all-purpose flour, baking soda, and salt. Make sure there are no lumps of baking soda, as that can lead to a weird taste in spots.
5. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until *just* combined. You'll want to stop mixing as soon as you no longer see any dry streaks of flour. Overmixing develops too much gluten, which can make your cookie slabs tough. A slightly shaggy dough is perfectly fine!
6. Now for the fun part: gently fold in all those glorious chocolate chips – the semi-sweet, milk, and white chocolate chips. Use a spatula to fold them in until they are evenly distributed throughout the dough. Try not to crush the chips.
7. Carefully transfer the cookie dough into your prepared 9x13 inch baking pan. Using your hands (lightly floured if it's too sticky) or a clean offset spatula, press the dough evenly into the pan. Make sure it reaches all the corners and is as flat as possible for even baking.
8. Pop the baking pan into your preheated oven and bake for 25-30 minutes. You're looking for the edges to be nicely golden brown, and the center should look set but still a little soft when you gently press it. Resist the urge to overbake, as that's the key to a gooey center! Every oven is a little different, so keep an eye on it.
9. Once baked, remove the pan from the oven and let the cookie slab cool completely in the pan on a wire rack. This is crucial! It allows the cookies to set up properly and become firm enough to cut without crumbling. If you cut it too soon, it’ll be a beautiful, gooey mess (which isn't bad, but might be hard to serve!).
10. Once fully cooled, use the parchment paper overhang to lift the entire cookie slab out of the pan and onto a cutting board. Slice it into squares or rectangles of your desired size. I usually go for 2x2 inch squares, but you do you! Serve them up and watch them disappear.