Spray an 8-inch square baking dish with nonstick cooking spray.
Combine the oat cereal and marshmallows in a large bowl.
Combine the butter, brown sugar, honey and salt in a large saucepan and bring to a simmer. Cook for 1 minute, then carefully stir in the vanilla. Add the cereal mixture and stir to coat completely.
Spread the mixture in the prepared baking dish and press into an even layer. Allow to cool and set, about 1 hour, before cutting into squares.