Preheat oven to 350 degrees F.

Heat a 6-9 inch oven-safe skillet over medium high heat. Add the butter and and allow to fully melt and start the browning process (note: I browned my butter in the pictured dark cast iron skillet. If you're never browned butter before, I would suggest browning the butter in a lighter skillet the first couple times to prevent burning the butter). Once the butter has melted, it with foam up and bubble. Keep a constant eye on the butter, stirring every few seconds with a rubber spatula. Once the bubbling decreases, little golden brown specks will start to appear at the bottom of the pan and the butter will start to small nutty. Immediately remove your butter from the heat and carefully transfer to a mixing bowl to prevent further cooking. Allow the butter to cool for 2-3 minutes.

Meanwhile, in a separate bowl, whisk together the flour, cornstarch, baking soda and salt until fully incorporated. Set aside.

Once butter has cooled slightly, whisk in both the dark brown sugar and the granulated sugar and mix until fully combined.

Add the egg and vanilla extract and whisk for 2-3 minutes or until mixture is fully incorporated. Stir in the Greek yogurt.

With a wooden spoon or rubber spatula, gently mix the dry ingredients with the wet ingredients, mixing only until just combined. Gently fold in the dark and milk chocolate chunks and the rainbow sprinkles.

Careful not to burn your hand on the skillet's handle, spread the cookie batter evenly into your skillet. Sprinkle the top with additional chocolate chips, rainbow sprinkles and sea salt (if desired). Bake for 15-25 minutes, or until edges are golden brown, and center is nearly cooked but slightly gooey.

Once cookie is baked to desired doneness, remove from oven and top with ice cream or wait 5-10 minutes, allowing the cookie to set, and slice like a pie.