Set 18 mini silicone muffin cups into a mini muffin pan. You can also use parchment cups, but they are trickier to work with.
In a heatproof bowl set over a pan of barely simmering water, melt the chocolate and cocoa butter together, stirring until smooth. Remove the pan from heat but keep the bowl over the pan.
Add about 1 teaspoon of the chocolate mixture to the bottom of each cup and use a small spoon to spread over the bottom and up the sides. Make sure each cup is well covered. Freeze until firm, about 15 minutes.
In a large saucepan over medium low heat, combine the marshmallows, butter, and water. Stir until melted and smooth. (If using ChocZero marshmallows, the mixture will be quite sticky and you will need to return to low heat every so often).
Spoon about 1 tablespoon of the marshmallow mixture into each cup and return to the freezer for at least 30 minutes.
Prepare the caramel sauce as directed in the recipe and let cool to room temperature. It should still be liquid but not hot. If you already have some prepared, re-warm it to a liquid consistency.
Spoon caramel sauce over the marshmallow, filling almost to the top of the cup. Return to the freezer for another 30 minutes.
Rewarm the remaining chocolate until melted and spoon over the top of each cup, spreading to cover the filling. Refrigerate or freeze until completely set.
Notes
Storage Information: Store in an airtight container on the counter for up to a week, or in the fridge for up to two weeks. You can also keep them frozen for several months. Homemade Marshmallow Option: I know ChocZero isn't available to everyone, so you do have the option to make your own keto marshmallows with my recipe. You will likely need no more than a half batch and you will want to chop them into small squares so that you can measure them out properly.