Line a baking pan with non-stick foil or parchment paper.
Grease a large bowl and add the cereal. Set aside.
In a microwave safe bowl, place the butter, cut in pieces, and the marshmallows.
Microwave for 1 minute and let sit a couple minutes. Stir very well until the mixture is completely smooth. If needed, add another 15 seconds in the microwave.
Alternatively, heat the marshmallows and butter in a saucepan over low, stirring. When 75% of the marshmallows have melted, remove from heat and stir, using residual heat to fully melt the marshmallows.
Fold the marshmallow mixture over the cereal and stir well.
Transfer to the prepared pan, gently pressing the treats evenly.
Let cool, 30 minutes to 2 hours, and cut into squares.
Notes
Store in an air-tight container up to 5 days, or freeze for 3 months. LARGE BATCH To make Kix treats in a 9 x 13 inch baking pan, use: