Line an 8x8-inch baking pan with parchment paper and lightly grease with non-stick spray.

In a medium saucepan over medium heat, combine peanut butter, corn syrup, and sugar. Stir constantly until the mixture is smooth and begins to bubble slightly, about 3-4 minutes.

Remove from heat and stir in vanilla extract and mini marshmallows until melted and combined.

Pour the peanut butter mixture over the Rice Krispies cereal in a large bowl. Stir gently until cereal is evenly coated.

Press the mixture firmly into the prepared pan using a greased spatula or your hands. Let cool completely before cutting into squares.