Prepare a 9x13-inch pan by lining it with parchment paper and lightly misting it with nonstick cooking spray.
In a large pot, melt the unsalted butter. Stir continuously until the butter starts to brown, foams up twice, and releases a nutty aroma. Remove from heat immediately to prevent burning.
Quickly stir in 2 teaspoons of vanilla extract. The mixture will bubble up, so keep stirring for an additional thirty seconds until the sizzling subsides. Add salt.
Mix in half of the marshmallows, allowing them to almost completely melt before adding the rest. Stir until all marshmallows are fully melted. Then, add the Rice Krispies and gently stir until combined. Finally, add the Lucky Charms and mix until incorporated.
Pour the mixture into the prepared pan. Lightly wet your hands and gently press it to the edges, being gentle to avoid crushing it.