Line a sheet pan with parchment paper. Spray a large pot and a rubber scraper with nonstick cooking spray. Set aside.: Right away you will notice a faint, neutral plastic aroma from the cooking spray, which signals that the surfaces are protected from sticking. This preparation step matters because marshmallow mixtures cling stubbornly to metal and wood, and a well lined pan ensures the shaped eggs lift off cleanly. A common pitfall is skipping the spray on utensils, which makes scraping the pot a slow chore and can waste marshmallow; take the minute to spray thoroughly so you can move swiftly later.
Place pot over medium heat and melt butter. Pour in 10 oz (one bag) of mini marshmallows and stir to coat in butter. Continue stirring until marshmallows have melted completely. Remove from heat.: As the butter begins to melt you will see it foam and smell its nutty richness, a sign that it is warming properly. When you add the mini marshmallows , they will soften and then become glossy and fluid. Stirring continuously keeps the heat even and prevents scorching, which would make the mixture grainy. Remove the pot as soon as the marshmallows are smooth to avoid overheating; if you keep it on the heat they can seize and become tough.
Pour in cereal and stir to coat in melted marshmallows. Once mostly coated, stir in the remaining 5 oz marshmallows. They will melt slightly from the heat, but not completely which is what you want.: When the Rice Krispies cereal hits the glossy marshmallow, the sound changes to a soft, shuffling scrape as each crisp grain gets enveloped. The initial stir ensures most pieces are covered, and folding in the extra mini marshmallows gives you textural islands of chew. The little marshmallow bits should look plump not dissolved; if they fully disappear you probably mixed them in too early or the pot was too hot, so adjust your timing next batch.
Working very quickly, also add in Cadbury Mini Eggs (saving a handful for later – optional, see note below) and stir until combined well.: At this point you will notice the chocolate shell warming slightly and the air filling with a faint cocoa scent. Stir fast so the Cadbury Mini Eggs are distributed before the mixture cools and firms. The reason for speed is to preserve candy texture; if you linger the shells may soften and bleed color into the marshmallow. A typical mistake is adding the candies too early while everything is still hot enough to dissolve them, which can make the treats blotchy instead of pretty.
Using lightly greased fingers, take a handful of the mixture and form into Easter Eggs. Place on baking sheet.: When shaping, the mixture should feel warm, tacky, and pliable. Press gently but firmly so the cereal binds together without squeezing out too much marshmallow. Use a light film of cooking spray on your hands to prevent sticking; if your hands are dry the mixture will crumble, and if they are too wet the surface becomes soggy. The visual cue is a smooth, rounded exterior with visible specks of candy, not a mass that falls apart.
If you saved some of the Cadbury mini eggs, strategically place them all around the egg treat for a nicer presentation (optional).: Adding reserved candy halves after shaping gives you control over placement, creating a polished look. The candies will sit on the surface rather than melt in, maintaining color and crunch. A common error is pressing the candies in too deep, which can displace structural marshmallow and create weak spots that crumble later; gently nudge them into place instead.
Allow to cool completely. Store in an airtight container for up to 3 days to maintain freshness.: As the treats cool you will hear the faint settling, and the mixture firms from tacky to holdable. Cooling fully helps the shape keep integrity when handled. Storing in an airtight container traps moisture and keeps the cereal crisp, while leaving them out too long exposed to air can make them stale. If you notice softness after storage, a short crisp in a low oven for a minute or two can revive texture, but typically these are best enjoyed within the first day.