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 100g marshmallows, 1 tbsp light brown sugar, and a pinch of salt, and melt together, stirring to prevent scorching, til marshmallows are fully melted and bubbling slightly. Cut the heat and add a tsp of vanilla extract.
Add 50g rice krispies (or 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 etc if desired and 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.)