Grease a 9x13 inch baking pan or line it with parchment paper.

In a large pot, melt the butter over medium-low heat. Add the fresh marshmallows and stir continuously until completely melted and smooth. Remove from heat immediately.

Stir in the vanilla extract and a pinch of salt.

Add the crispy rice cereal and fold gently until all the cereal is evenly coated with the marshmallow mixture.

Transfer the mixture to the prepared baking pan. Using a buttered spatula or wax paper, gently press the mixture into an even layer. Do not pack it down tightly. Let set at room temperature for 30 minutes.

Remove the set cereal block from the pan. Trim the edges to make them straight, then cut the block into uniform 2x3 inch rectangles.

Melt one color of candy melts in a microwave-safe bowl at 50% power in 30-second increments, stirring until smooth. Add a tiny bit of coconut oil if the mixture is too thick.

Spread an even layer of the melted candy coating over the top of a Rice Krispie rectangle.

Immediately place 6 M&M candies of the matching color onto the wet coating in two rows of three, with the 'M' facing down.

Repeat with the remaining colors and treats. Let sit at room temperature for 20 minutes until the candy coating is fully hardened.