Melt butter in a large non-stick saucepan over medium low heat, until lightly browned and nutty, about 5-8 minutes.
Turn heat down to low. Stir in marshmallows, and continue to stir until totally melted.
Turn off the heat and stir in ½ teaspoon kosher salt.
Add the Rice Krispies and stir just until everything is coated.
Scrape mixture into prepared baking pan.
Coat a rubber spatula with cooking spray and gently press the mixture into an even layer. Press lightly, don't pack it down, or you'll end up with dense, hard treats instead of gooey ones.
Let cool for at least 30 minutes, then remove from pan and cut into squares.
Notes
Store in an airtight container at room temperature (with waxed paper between layers) for up to 3 days. Freeze pre-cut squares for 4-6 weeks.Cook the butter low and slow. You’re looking for a light golden color and a nutty aroma, with some little brown flecks at the bottom of the pan. Contrary to what you might expect, browning the butter more will not improve the flavor.