Line a 9×13 pan with parchment paper and lightly spray with nonstick cooking spray. Set aside.
In a large pot, melt the butter over medium heat. Keep stirring until the butter starts to brown slightly. It should foam up twice and then give off a nutty aroma. Remove from heat immediately to avoid burning.
Stir in the vanilla extract (watch out for the sizzle!), then add the salt.
Add half of the marshmallows and stir until almost melted, then add the rest. Keep stirring until all marshmallows are melted.
Add the Rice Krispies and gently stir until combined. Do the same with the Lucky Charms.
Pour the mixture into the prepared pan. With wet hands, gently press it into the pan without crushing it.