To help in forming the cone shape, I took the foam that comes in a waffle cone pack to protect the cones and cut it to make a form to help shape the trees.
Line a cookie sheet with parchment paper.
In a medium to large saucepan, melt the butter over low heat.
Add the marshmallows and stir until completely melted.
Add enough food coloring to make mixture dark green (it will lighten when cereal is added).
Remove from heat.
Add the Rice Krispies cereal and stir until cereal is well coated.
Allow to set about 4 minutes.
Butter a your hands and form mixture into cone shapes.
Place cones onto prepared pan.
Allow to cool and firm up for several hours.
Mix powdered sugar, milk and corn syrup in a bowl until smooth.
Transfer to piping bag or zippered bag to pipe onto cookies.
Pipe like a garland around each tree.
Space M&M minis like ornaments onto the garland.
Pipe a dab of icing on top of tree to act as glue and attach star.
Place a Reese's upside down on pan. Pipe icing on it and place tree on top.
Allow to set before serving.