Line 9 x 9 pan with heavy aluminum foil.

In a double boiler, melt chocolate and butter over medium heat, stirring occasionally, until thoroughly combined, remove from heat and stir in marshmallows.

Scrape chocolate mixture into prepared pan using a silicon spatula; smooth into a somewhat even layer.

Refrigerate until chocolate sets, at least 1 hour. Keep refrigerated until ready to eat.

Cut into 12 pieces and serve.