Line a small baking pan with parchment paper and set it aside.
Melt the butter in a large saucepan over medium heat. Add the mini marshmallows and stir until fully melted and smooth.
Remove the pan from heat. Stir in the Rice Krispies cereal until evenly coated.
Add 1/2 cup of the dark chocolate chips to the warm mixture and stir until they melt and distribute through the cereal.
Press the mixture firmly into the prepared pan in an even layer. Let it cool for 20 minutes.
Cut the cooled slab into four bat wing shapes using a sharp knife.
Combine the remaining 1/2 cup dark chocolate chips with the coconut oil in a microwave-safe bowl. Heat in 20-second bursts, stirring between each, until smooth.
Drizzle the melted chocolate over the cut bars and let it set at room temperature.