📖 Instructions

1. Before diving in, make sure your oven is preheated to 350°F (175°C). This is super important for an even bake, so give it about 15-20 minutes to get up to temp. While that’s happening, grab a 9x9 inch square baking pan and line it with parchment paper, leaving a slight overhang on two opposite sides. This little trick makes lifting the cooled cookie out a breeze, trust me.
2. Now for the wet ingredients! In a large mixing bowl, either with a stand mixer fitted with the paddle attachment or just a good old hand mixer, cream together the softened unsalted butter and both the granulated sugar and light brown sugar. Beat 'em together on medium speed until the mixture is light, fluffy, and looks like a pale, glorious cloud, about 2-3 minutes. Don't rush this step; it incorporates air, which is key to a soft cookie.
3. Next, crack in your large egg. Make sure it's at room temperature – this helps it combine better with the butter and sugar. Beat it in until it’s just combined. Then, add the egg yolk (the extra yolk adds richness and chewiness!) and the pure vanilla extract. Beat again until everything is fully incorporated, scraping down the sides of the bowl with a spatula to ensure no bits are left behind.
4. Time for the dry bits! In a separate, medium-sized bowl, whisk together the all-purpose flour, baking soda, and salt. A good whisk here ensures everything is evenly distributed. Slowly add this dry mixture to your wet ingredients, mixing on low speed until *just* combined. The trick is to stop mixing as soon as you see no more streaks of flour. Overmixing develops the gluten too much, leading to a tough cookie, and we're aiming for gooey and tender!
5. Here comes the fun part: the chocolate! Fold in the semi-sweet chocolate chips and the chopped milk chocolate bar. A spatula is your friend here. Gently fold until the chocolate is evenly distributed throughout the dough. You want a good mix, but again, don't overdo it. The melted chopped chocolate adds an extra layer of gooey decadence that you just won't get from chips alone, it's a game-changer!
6. Press the cookie dough evenly into your prepared 9x9 inch baking pan. Use the back of a spoon or your clean hands to press it down firmly and smoothly into all the corners. Make sure it's an even layer, so it bakes consistently. Take your time here to get it right; a uniform thickness means a uniform bake.
7. Pop that pan into your preheated oven. Bake for about 28-32 minutes. Now, this is crucial: you want the edges to be set and lightly golden brown, but the center should still look slightly soft and underbaked. This is the secret to that magical gooey center! If it looks completely set in the middle, you’ve probably gone too far and you'll miss out on some of that glorious melty goodness. Err on the side of slightly underbaked.
8. Once out of the oven, do not, I repeat, do not try to cut them right away! That gooey center needs a moment. Let the cookie cool completely in the pan on a wire rack for at least 1 to 1.5 hours. I know, I know, the wait is excruciating, but it allows the cookie to set properly, making it easier to cut and ensuring that perfect texture. If you try to cut it too soon, it’ll be a crumbly mess.
9. After cooling, use the parchment paper overhangs to carefully lift the entire cookie out of the pan and transfer it to a cutting board. Use a sharp, non-serrated knife to cut the large cookie into 9 or 12 squares, then cut each square diagonally to form those beautiful wedges. You can also cut into 16 squares and then cut them diagonally for smaller wedges, if you're feeding a big crowd. Serve them as is, or for an extra special treat, warm them slightly in the microwave for a few seconds to bring back that fresh-from-the-oven gooeyness. Enjoy every single delicious bite!