Grease 2 3" ramekins or similar form (or, ready a pair of deli containers, which do not need to be greased.)
Slowly (so as not to scorch it) melt 2 tbsp butter. Add marshmallows and a pinch of salt and stir continuously til marshmallows are fully melted and bubbling slightly. If you cut the heat the moment the marshmallows start to brown on the bottom of the pan, the residual heat should carry it from there.
Add 50g rice krispies *(r other cereal based on volume not weight) and stir til combined.
Split into containers and press down. (If using deli containers, you can use the bottom of another deli container to press them into shape.)
Top with mini m&ms. press (firmly, but once) into the top to stick. Allow to cool to solidify (or not.)
Can optionally by topped with fluff and/or broiled. (Do not broil deli containers.)