In a large pot over medium heat, melt the butter, stirring frequently, until it turns golden brown and smells nutty.
Reduce the heat to low and stir in the mini marshmallows and kosher salt until fully melted and smooth. Remove from heat and mix in the vanilla extract.
Gently fold in the Lucky Charms cereal, stirring until evenly coated with the marshmallow mixture.
Transfer the mixture to a 9x13-inch pan lined with parchment paper or greased with nonstick spray. Lightly press it into an even layer using a rubber spatula or butter wrappers (avoid pressing too firmly for softer treats).
Sprinkle with flaky sea salt, if using. Let the treats cool completely at room temperature before slicing and serving.
Notes
Use fresh cereal – Seriously, don’t use an old box of Lucky Charms from the back of the pantry. Fresh cereal = the best texture for these lucky charms rice krispies treats.
Melt butter slowly – Medium heat is best to get that golden, nutty brown butter without burning it.
Don’t press too hard – Lightly press the cereal mixture into the prepared pan so they stay chewy instead of dense.
Line your pan for easy removal – A piece of parchment paper or wax paper makes lifting them out so much easier!
Let them cool completely – I know it’s hard to wait, but if you cut them too soon, they’ll fall apart.