Preheat your oven to 350 degrees F (175 degrees C). This is a crucial step as it allows your cookies to bake evenly. While the oven heats up, grab a 10-inch cast iron skillet and spray it with cooking spray to prevent sticking.
In a medium-sized bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of baking soda, and 1/2 teaspoon of kosher salt. This mixture will help ensure even distribution of the leavening agent throughout your dough.
In a large bowl, add 1 cup of unsalted butter, 1/2 cup of dark brown sugar, and 1/2 cup of white granulated sugar. Using a mixer, cream the butter and sugars together until the mixture is light and fluffy, which should take about 3 minutes. This step is key to achieving a light texture in your cookies.
Next, add in 1 large egg and 1 1/2 teaspoons of pure vanilla extract. Mix until they are thoroughly combined. You should see a smooth, creamy texture.
Gradually add the flour mixture to the wet ingredients in two batches. Stir gently until just combined, ensuring that you avoid overmixing, which could lead to tougher cookies. You want to see flecks of flour without any visible dry spots.
Now, fold in the chopped Cadbury Mini Eggs gently until they're evenly distributed throughout the dough. The vibrant colors should be a delight to see!
Pour the cookie dough into your prepared skillet, flattening it into an even layer. This will help the cookie bake evenly and achieve that delicious golden brown color.
Transfer the skillet to the oven and bake for 20 to 22 minutes. Keep an eye on it; you want the edges to turn golden brown while the center remains soft to the touch. Don't worry if it looks a little underbaked; it will set as it cools.
Once baked, remove the skillet from the oven and allow the cookie to cool completely in the skillet. Patience is key here! You can cut it into slices once it's cool. If you can’t wait, it will just be super soft and crumbly, but that’s not the worst thing!
Serve these delicious cookie slices on a plate with glasses of cold milk or enjoy them as a delightful snack any time of the day.