Preheat your oven to 350°F (175°C). This ensures that your cookies will bake evenly and achieve that perfect texture.
In a large mixing bowl, combine the room temperature unsalted butter, brown sugar, and granulated sugar. Using a whisk or an electric mixer, cream the mixture until it is light and fluffy, about 2-3 minutes.
Add in the egg, egg yolk, and vanilla extract to the butter-sugar mixture. Continue to mix until well combined and smooth.
In a separate bowl, sift together the all-purpose flour, unsweetened cocoa powder, baking soda, salt, and ground red pepper. This helps to evenly distribute the dry ingredients and ensures a smooth batter.
Gradually add the dry mixture to the wet ingredients, mixing just until combined. Be careful not to overmix, as this can lead to tough cookies.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto a prepared baking sheet lined with parchment paper, leaving space between each cookie. Bake in the preheated oven for 10-12 minutes, or until the edges are set but the centers are still soft.
Once baked, remove the cookies from the oven and allow them to cool on the baking sheet for 5 minutes before transferring them to a cooling rack. Enjoy your Spicy Chocolate Cookies warm, or store them for later!