Prepare a 9x13-inch baking dish with a coating of butter or nonstick cooking spray.
Place the cereal in a very large mixing bowl and set aside.
Melt the butter in a large, light-colored saucepan over low heat, stirring occasionally.
Split the vanilla bean lengthwise and scrape out the seeds with the back of a knife. Add the seeds (and pod, if you like) to the melted butter and continue cooking over low to medium-low heat.
The butter will foam, bubble, and crackle as the water cooks off. Keep stirring and watch the color at the bottom of the pan.
After about 5-7 minutes, the bubbling will quiet down, brown specks will form on the bottom, and the butter will turn a deep golden brown and smell nutty. As soon as it reaches this stage, take the pan off the heat to prevent burning.
Remove the vanilla pod and immediately add the marshmallows and salt to the hot browned butter.
Stir over low heat until the marshmallows are completely melted and the mixture is smooth, about 3-5 minutes.
Pour the melted marshmallow mixture over the cereal. Fold gently with a spatula to coat evenly.
Using a buttered spatula or lightly greased hands, gently press the mixture into an even layer. Don’t pack it down too hard or the treats will be too dense.
Let the bars cool at room temperature until set, 30-60 minutes, before cutting into squares and serving.