Preheat oven to 350°F.

On low-medium heat, heat a 12" cast iron skillet. Meanwhile get the rest of the ingredients together.

Add butter to skillet and melt (you may brown the butter at this stage for extra flavor). If you're not going to brown the butter, remove skillet from heat about 3/4 of the way through melting the butter. Then whisk until fully melted. This reduces the amount of cooling time.

Remove from heat and let cool for 10 minutes.

Stir in sugars. Mix with a wooden spoon.

Add eggs and stir until well incorporated (This is why butter needs to cool. No scrambled eggs!)

Add vanilla extract. Stir again.

Add flour, baking soda, baking powder and salt. Gently stir until well mixed. Use a rubber spatula to scrape down flour off the sides of the cast iron.

Add chocolate of choice and gently fold in.

Spread the batter evenly throughout the cast iron.

Bake skillet for 20-25 minutes, or until mostly set in the center. You can do less time if you want it super gooey.

Top with flakey sea salt (Maldon is my favorite)

Let cool slightly (if you can wait), then dig in. Serve while warm with vanilla ice cream and more chocolate syrup.