Preheat the oven to 400°F.
Add the semi-sweet chocolate chips, milk chocolate chips, butter, heavy cream, vanilla extract, and salt to an 8-inch cast-iron skillet or small oven-safe baking dish.
Place the skillet over low heat on the stove.
Stir often until the chocolate melts smooth.
Remove the skillet from the heat.
Spread the chocolate mixture into an even layer.
Sprinkle the mini marshmallows across the top in one even layer.
Bake for 5 to 7 minutes.
Watch closely near the end so the marshmallows toast without burning.
Broil for 30 to 60 seconds if you want deeper browning.
Stay near the oven while broiling.
Let the dip rest for 3 to 5 minutes before serving.
Serve warm with graham crackers, pretzel rods, strawberries, or cookies.