Place the butter in the top of a double boiler set over gently simmering water. Stir until melted and smooth. No double boiler? Set a heat-safe bowl over a pot of simmering water, keeping the bowl above the water.

Add the chocolate chips and stir until melted. Remove from the heat. Let it cool for 10 minutes at room temperature, stirring occasionally.

Add marshmallows to the melted chocolate and stir to coat the marshmallows fully.

Place two 15×18-inch pieces of parchment paper on a flat surface. Place half of the chocolate-marshmallow mixture onto the middle of each piece of parchment paper and shape it into a 12-inch long log using a rubber spatula.

Wrap the logs tightly in the parchment and refrigerate for at least 4 hours.

After four hours or more, cut into 1/2-inch thick slices.