Lightly coat a 9x13" pan with cooking spray.
In a large saucepan, melt butter over low heat. Once melted, add in marshmallows. Stir until the marshmallows are completely melted.
Remove saucepan from heat and add Rice Krispies to the melted marshmallows. Add molasses, vanilla, ginger, cinnamon, allspice, nutmeg, and salt and gently stir mixture until cereal is completely coated in marshmallow.
Spoon the mixture into the prepared pan and gently press down. I recommend using a lightly oiled rubber spatula so it's not too sticky. Don't press too compactly as you want treats to still be light and airy.
Let treats cool for about 5 minutes before using a lightly oiled gingerbread man cookie cutter to cut out shapes. Press remaining scraps together in the pan and continue cutting shapes until all of the. mixture is gone.
Use mini M&Ms and melted white chocolate to decorate gingerbread men, if desired. You can use the melted chocolate to adhere the M&Ms.