Line an 8-inch square pan with aluminum foil or parchment. Spray with nonstick cooking spray or spread with butter.
Melt butter in a large glass bowl. Add the marshmallows and salt. Continue cooking in the microwave, stirring every 30 second with a rubber spatula, until melted.
Add the vanilla, then the cereal. Stir until evenly coated.
Press the cereal mixture into the prepared baking pan with wet hands. Let cool for about 10 minutes.
Use the foil to remove from the pan. Place on a cutting board and cut into squares.
Notes
After the marshmallows start to melt, watch closely. The mixture can quickly burn.
Gently press the cereal mixture into the pan. If you press down too firmly, the bars will be dense and hard to bite into.
Leftover bars can be stored at room temperature in an airtight container for 2-3 days. They start to soften after that, but are still tasty. For maximum freshness, wrap them individually in plastic wrap.