Lightly butter an 8-inch square pan.
In a large pot, melt the butter over very low heat. Stir in the marshmallows, getting them all coated with the butter. Slowly melt the marshmallows, stirring frequently. When the marshmallows are melted and smooth, remove the pan from the heat. Stir in the instant coffee (2 tablespoons for a rich coffee flavor, 3 tablespoons for a real kick) and mix until it is dissolved into the melted marshmallow. Gently fold in the rice cereal. When it is completely coated, lightly press the mixture into the prepared pan with your buttered hands. When cool, cut the treats into bars and serve.