Line a 9×13-inch baking pan with parchment paper and lightly spray it with nonstick cooking spray. Set aside.

In a large pot over medium-low heat, melt the butter. Once fully melted, stir in the vanilla extract. Continue stirring for an additional 30 seconds, allowing the mixture to bubble slightly before settling.

Add half of the marshmallows to the pot and stir until nearly melted. Add the remaining marshmallows and continue stirring until fully combined and smooth. If necessary, reduce heat to low to ensure even melting.

Remove the pot from heat. Gradually add the Fruity Pebbles cereal, two cups at a time, and gently fold the mixture until evenly coated with the marshmallow mixture.

Pour the mixture into the prepared pan. Using lightly dampened hands or a silicone spatula, gently press the mixture into an even layer. Avoid compressing it too firmly to maintain a light, chewy texture.

Place the white chocolate in a heatproof bowl. Microwave in 30-second intervals, stirring after each, until fully melted. Allow the chocolate to cool slightly for 2–3 minutes before use.

Drizzle the melted white chocolate over the pressed cereal mixture. Sprinkle with nonpareils and additional Fruity Pebbles for garnish.

Allow the treats to cool completely for at least 30 minutes. Once set, slice into squares or bars and serve.