Gather all ingredients and tools (a large mixing bowl, spatula, and a greased 9x13-inch pan).
Melt butter in a large saucepan over low heat.
Add marshmallows to the melted butter, stirring until completely melted and smooth.
Remove from heat and add Rice Krispies cereal to the marshmallow mixture.
Stir quickly until all cereal is evenly coated.
Pour the mixture into the greased pan.
Use a spatula to press the mixture evenly into the pan without compacting it too much.
Allow it to cool at room temperature for about 30 minutes.
Once set, cut into squares or rectangles.
Optionally, drizzle melted chocolate and sprinkle with decorations before serving.