Microwave butter and marshmallows in a large microwaveable bowl for 2 minutes on high. Stir, continue heating in 30 sec. increments until fully melted and combined.
Add crisp cereal to the mixture, stir.
Place rice mixture onto a parchment lined jelly roll pan. Spread with a spatula until flat. (mixture should fill about 1/2 a standard size pan at 1" thick).
Fold the excess parchment paper onto the rice krispy treats. Using your hands or a rolling pin continue to work the mixture until it is completely flat.
Place mixture in the freezer for 5 minutes to set.
Cut out Mickey shapes with a cookie cutter. Place treats back in the freezer while you prep the chocolate.
Melt dark chocolate according to package instructions. Dip just the ears into chocolate. Place back onto parchment and add sprinkles. Set chocoalte in freezer for at least 10 minutes.
For optimal serving, treats should sit at room temperature for 10 minutes to soften. Keep leftover treats in an airtight container in the fridge. Freeze up to 6 weeks.