Preheat oven to 350℉ (180℃)
Lightly brush each cast iron skillet with melted butter.
2 Tablespoons Unsalted butter
In a medium bowl, sift together the flour and cocoa powder to remove any lumps.
½ cup All-purpose flour, ⅓ cup Cocoa Powder
Place chocolate and butter in a heat-proof bowl. Microwave at 50% power for 30 seconds. Stir well. Continue heating for 30 second bursts at 50% power until melted, stirring well between each heating. Cool slightly.
1 cup Chocolate chips, ½ cup Unsalted butter
Add the egg, egg yolks, sugar and vanilla extract to the bowl with the chocolate and butter. Whisk together until well combined.
2 large Egg yolks, 1 large Egg, ⅔ cup Granulated sugar, 1 teaspoon Pure vanilla extract
Add the flour and cocoa mixture to the mixing bowl. Stir until well combined and no dry ingredients are visible.