Lightly coat a 9x13-inch baking pan with nonstick cooking spray and set aside.
In a large, heatproof bowl, combine the Rice Krispies and 1 cup of mini marshmallows. Set aside.
In a large, heavy-bottomed saucepan, melt the butter over medium-high heat, stirring constantly. Once melted, reduce the heat to medium and add the brown sugar, whipping cream, and corn syrup. Cook for about 7 minutes, stirring frequently to prevent scorching.
When the caramel is thick and syrupy, stir in the vanilla extract and salt, then remove from the heat and add green food coloring and mix until evenly tinted. Stir in marshmallows, stirring constantly until completely melted.
Quickly pour the mixture over the Rice Krispies and stir until everything is evenly coated. Press the mixture firmly into the prepared pan, then decorate the top with candy eyes and Halloween-colored sprinkles for a fun monster look.
Allow the treats to cool completely before cutting into squares.