Butter a 9 x 13 inch (23x33 cm) baking dish and set aside.

In a large microwavable bowl add the marshmallows and butter. Microwave for 1 minute at a time, stirring in between heatings, until the marshmallows have fully melted. (This step can also be done on the stove top in a large pot over low heat, stirring constantly.)

Using a greased spatula, fold in the Fruity Pebbles, vanilla and salt until the cereal is evenly coated.

Spread into your prepared baking dish, and press down gently to create an even, solid layer.

Place the rice crispy treats on the counter for 30 minutes to set. Cut into 12 squares.

Store in an airtight container at room temperature for up to 3 days.