Spray a 9 by 13 inch pan with cooking spray.
In a large saucepan, melt 3 TBSPs butter. Once melted add the marshmallows stirring regularly until melted. Add the cinnamon and nutmeg until well distributed. Remove from the heat and stir in the Rice Krispies®, stirring so they are fully coated. Finally, add the shredded carrots and pour into the 9 by 13 inch pan. Push down and allow to fully cool.
Beat together the cream cheese, butter, powdered sugar, and vanilla extract until smooth. Spread over the cooled Rice Krispies® Treats and serve.