Place the unsalted butter, vegetable oil, unwrapped caramels, and miniature marshmallows in a large microwave-safe bowl.
Microwave the mixture in 45-second intervals, stirring thoroughly after each interval, until every ingredient has melted into a smooth, uniform coating. The process will generally require 2–3 minutes, depending on the microwave.
Transfer the popped popcorn to a large mixing bowl. Pour the warm caramel mixture evenly over the popcorn.
Using a rubber spatula, fold the ingredients together gently but thoroughly until the popcorn is evenly covered. Avoid excessive pressure to prevent crushing the popcorn.
Allow the coated popcorn to cool for 1–2 minutes so the candy shells do not melt excessively.
Add the M&M’s and stir only until they are distributed throughout the mixture. Overmixing may cause the candy colors to streak.
Generously grease a Bundt pan, then spoon the popcorn mixture into it.
Press the mixture down lightly and evenly to form a compact ring. Leave it at room temperature for approximately 15–20 minutes, or until completely cool and firm.
Invert the Bundt pan onto a serving platter and carefully release the set popcorn cake.
Cut the popcorn ring into portions and serve.