Add chocolate chips and coconut oil to a microwave safe bowl and microwave in 30 second increments until melted, stirring as you go. Stir in protein powder until fully blended with coconut oil/chips mixture.
Using a silicone muffin pan, fill each cavity with a small amount of the chocolate mixture to cover the bottom and use a small spoon to bring the mixture up the sides to create a cup. Place the in the freezer for about 5 minutes to set.
While the chocolate is setting, prepare the filling by combining the nut butter and mashed banana in a small bowl, mixing until smooth.
Divide the mixture evenly between your cups (about 12 mini or 6 regular) and cover with the remaining melted chocolate mixture. Use a spoon to smooth out the top, and then freeze for a minimum of 10 minutes for everything to set.
Store in an airtight container in the fridge for up to a week or in the freezer for longer periods.