Spray a Bundt pan generously with non-stick cooking spray. Set aside.
Melt butter in a large saucepan over medium heat. Add marshmallows. Stir until melted and completely combined. Add cereal and stir until it's combined with the marshmallow mixture. Quickly pour into the prepared pan. Press it down into pan with greased fingers or a greased spoon. Smooth top so it's level. Let cool about 1 hour.
Invert cake onto a large plate. Melt red candy melts in a small microwave safe bowl with the microwave set on high for 30 seconds. Stir, then heat another 30 seconds (if needed). Stir until smooth. Pour into a disposable plastic pastry bag. Cut the tip of the bag to create a very small hole. Drizzle red candy melts over cake. Repeat process with remaining candy melts. Add sprinkles to the top right away before the candy melts set. Slice and serve. Store covered at room temperature for 24 hours or in the refrigerator for a few days.
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Notes
If you don't have butter, you can substitute either shortening or vegetable oil in the same amount.