Preheat your oven to 350°F (175°C). Lightly grease the inside of a 10-inch cast-iron skillet with a little butter or non-stick baking spray.

In a microwave-safe bowl, combine the unsalted butter and semi-sweet chocolate chips. Heat in 30-second intervals, stirring smoothly, until completely melted and glossy. Set aside to cool slightly.

In a large bowl, vigorously whisk the eggs and granulated sugar together for 2 to 3 minutes until pale and slightly frothy. (This creates the crinkly top!)

Whisk the melted chocolate mixture and the vanilla extract into the eggs and sugar until smooth.

Add the all-purpose flour, cocoa powder, and salt. Use a stainless steel spatula to gently fold the dry ingredients into the wet batter just until no dry flour remains. Do not overmix.

Pour the batter into the prepared cast-iron skillet and spread it evenly to the edges.

Bake on the center rack for 22-26 minutes. A toothpick inserted 2 inches from the edge should be clean, while the center should still have moist, fudgy crumbs.

Remove the skillet from the oven and let it cool on a wire rack for at least 30 minutes before slicing, or serve warm with ice cream!