Preheat oven to 350 degrees F. Spray a 24-cup mini muffin pan with cooking spray. Set aside.
Combine crushed graham crackers, melted butter and powdered sugar together. Spoon one Tablespoon of mixture into each mini muffin cup; pressing down with the back of the measuring spoon to create a bowl shape.
Bake for 5 minutes.
Remove from oven and immediately place a chocolate square into each of the graham cracker crusts. Top each chocolate with one half of a marshmallow. Return pan to oven and bake for 2 more minutes. Be careful to watch the entire time to prevent burning.
Let S'mores Cups cool for 10 to 12 minutes in pan, then carefully transfer to a wire rack to cool completely. (Be very careful and gentle as you remove them to prevent from breaking. I use a butter knife and loosen around the edges then gently pop them out.)
In a medium, microwave-safe bowl, add the milk chocolate chips and shortening. Microwave in 25 second increments until barely melted. Stir well then place melted chocolate in a sandwich bag. Snip one corner off bag and then drizzle chocolate all over the tops of cups. Let chocolate set for about an hour before serving then enjoy!