Grease a 9x13-inch baking dish with butter or cooking spray.

In a large saucepan over low heat, melt the butter completely.

Add the mini marshmallows to the melted butter and stir continuously until fully melted and smooth.

Remove from heat and stir in the vanilla extract and a pinch of salt.

Gradually add the Rice Krispies cereal to the marshmallow mixture and stir until well coated.

Pour the mixture into the prepared baking dish and press down evenly with a spatula or greased hands.

Sprinkle colored sprinkles or toppings over the warm mixture, pressing gently to adhere.

Allow to cool completely at room temperature for about 30 minutes, then cut into squares or rectangles.