Line a 9x13 baking sheet with parchment paper. Extend the paper above the pan, and grease it with nonstick cooking spray. Set aside. Pull out about ¼ cup of the marshmallows from the cereal. You will add this to the top of the bars separately.
In a large pot, melt the butter over medium heat. Once the butter is melted, add the marshmallows and stir constantly until melted and smooth. Remove from heat.
Add Lucky Charms cereal to the marshmallows and stir well until fully incorporated.
Pour mixture into the prepared 9" x 13" pan. Use a greased spoon or spatula to spread the mixture into an even layer. Scatter the remaining marshmallows that you pulled out from the cereal and gently press them into the top of the bars to secure them a bit.
Allow the bars to fully cool down and set (about 30 minutes to an hour.) Pull bars out of the pan by lifting the parchment paper. Cut the bars into even pieces.
Melt the chocolate in 30-second increments in the microwave, stirring between every 30-second increments. Continue until fully melted. Dip the bottoms of the bars into the chocolate, making sure to come up the sides of the bars just a bit so you can see them from the sides of the bars.