Grease an 8x8 inch pan and line it with parchment. In a large pot over low heat, melt the butter. Add the marshmallows and stir constantly until almost melted, about 3 minutes. Remove from heat, then stir in the vanilla and salt.

Immediately fold in the Lucky Charms cereal with a silicone spatula until evenly coated.

Press the mixture into the prepared pan in an even layer. Cool at room temperature for 20 minutes, then lift out and slice into 16 squares.