📖 Instructions

1. Get Ready! Preheat your oven to 375°F (190°C). Line two large baking sheets with parchment paper. This is key for easy cleanup and preventing stickage, nobody wants stuck cookies.
2. Cream the Good Stuff. In a large mixing bowl, using an electric mixer, cream together the softened unsalted butter and both granulated sugar and light brown sugar until the mixture is light, fluffy, and looks almost pale. This usually takes about 2-3 minutes. Scrape down the sides of the bowl often! Don't skip this, it's how you get that wonderful cookie texture.
3. Egg-cellent Addition. Beat in the large egg and the egg yolk one at a time, mixing well after each addition. Then, stir in the pure vanilla extract. Make sure everything is well combined and smooth.
4. Dry Ingredients Unite. In a separate, medium-sized bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and a pinch of salt. Don't worry about sifting, just a good whisk will do to break up any lumps.
5. Combine 'Em Up. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until *just* combined. Be careful not to overmix here! Overmixing develops too much gluten, which can make your cookies tough. We're aiming for tender.
6. Divide and Conquer. Divide the cookie dough roughly in half into two separate bowls.
7. Red Velvet Magic. To one half of the dough, add the red food coloring gel (start with 1 teaspoon, add more if you want a deeper red) and the white vinegar. Mix until the color is evenly distributed and vibrantly red. The vinegar helps activate the cocoa and baking soda for that classic red velvet tang and color. For the other half of the dough, you can leave it plain or add another 1/2 teaspoon of vanilla extract for a more pronounced vanilla flavor.
8. Swirl Time! Take small, roughly equal-sized pieces of both the red and the plain dough. Gently roll them together in your hands to form a single ball, then lightly twist or fold them together a few times to create that beautiful marbled, swirled effect. Don't overdo it, or the colors will blend too much! We want distinct swirls.
9. Shape and Chill. Place the swirled dough balls about 2 inches apart on your prepared baking sheets. For a neater look, you can slightly flatten the tops of each dough ball. Pop the baking sheets into the fridge for at least 15-20 minutes. A little chill time helps prevent excessive spreading.
10. Bake Away! Bake the cookies for 9-12 minutes, or until the edges are lightly golden and the centers are still soft-looking. They might look a little underdone, but trust me, they’ll set up as they cool. Overbaking leads to dry cookies, and nobody wants that.
11. Cool Down. Let the cookies cool on the baking sheets for about 5 minutes before carefully transferring them to a wire rack to cool completely. This helps them firm up without breaking. Try not to eat them all while they're warm, though it's mighty tempting!