Step 1: Preheat your oven or broiler. If using an oven, preheat to 375°F (190°C). If using a broiler, preheat to high.
Step 2: In a 9-10 inch oven-safe skillet (cast iron works best), combine the semi-sweet chocolate chips, milk chocolate chips, butter, vanilla extract, and sea salt.
Step 3: Place the skillet in the preheated oven for 5-7 minutes, or until the chocolate begins to melt. Alternatively, you can melt the chocolate over low heat on the stovetop until smooth, then transfer to the oven for marshmallow toasting.
Step 4: Carefully remove the skillet from the oven. Stir the melted chocolate until smooth and evenly spread across the bottom of the skillet.
Step 5: Arrange the marshmallows evenly over the melted chocolate layer. You can pack them fairly tightly.
Step 6: Return the skillet to the oven or place it under the broiler. If using the oven, bake for another 5-8 minutes, or until the marshmallows are golden brown and puffed. If using the broiler, watch very carefully, as marshmallows can burn quickly – typically 1-3 minutes. Rotate the skillet if necessary for even toasting.
Step 7: Once the marshmallows are beautifully toasted to your liking, remove the skillet from the oven. Let it cool for a few minutes before serving to avoid burns.
Step 8: Serve warm directly from the skillet with graham cracker squares, fresh fruit, or even shortbread cookies for dipping. Enjoy your delicious Toasted Marshmallow S'mores Skillet Dip!