Line a large sheet pan with parchment paper and set to the side.
In a large saucepan over medium heat, melt the salted butter. Reduce the heat slightly and add the marshmallow fluff to the saucepan.
Stir over medium low heat until the marshmallow fluff breaks down and everything combines into a smooth liquid, about 1-2 minutes. Whisk in the orange food coloring until you reach the desired color and then remove from the heat.
Pour the rice krispies cereal into the saucepan and gently stir everything together until fully combined. Pour the orange rice krispie mixture out onto the baking sheet and spread into a thin layer. Set the mixture to the side and allow it to cool for about 20 minutes while you prepare the rest of the ingredients for decorating.
Separate out 12-14 chocolate baking chunks to be the stems and cut the M&Ms into even halves to be the leaves.
Once the rice krispie mixture has cooled enough that it will stick together when pressed, gently scoop it into a large mixing bowl. Spray an ice cream scoop lightly with cooking spray and use it to press the rice krispie mixture into large balls.
Spray an ice cream scoop lightly with cooking spray and use it to press the rice krispie mixture into large balls. Roll each scoop into a large, even ball and then place it back on the parchment paper lined baking sheet.
Microwave the remaining ½ cup of chocolate baking chunks for 30 seconds in a microwave safe bowl and then stir well. Place back in the microwave and continue heating in 15 second intervals, stirring after each heating, until the chocolate is completely melted.
Dip each chocolate baking chunk into the melted chocolate and then gently press it into the top of one of the rice krispie pumpkins. Place two green M&M halves next to the “stem” for leaves. Allow the pumpkins to set for another 10-20 minutes before serving.