Prepare the Pan: Line a 9x13-inch baking dish with parchment paper or lightly grease it with butter.
Melt the Butter and Marshmallows: In a large pot, melt butter over low heat. Add mini marshmallows and stir continuously until fully melted and smooth. Stir in vanilla extract if using.
Mix in Cereal: Remove the pot from heat and quickly add the Fruity Pebbles cereal. Stir gently but thoroughly to coat the cereal with the marshmallow mixture.
Press into the Pan: Transfer the mixture into the prepared baking dish. Use a spatula (lightly greased) to press it down evenly.
Cool and Set: Let the treats cool at room temperature for about 1 hour until firm.
Cut and Serve: Once set, cut into squares and enjoy!