Grease a 9X9-inch pan with butter. Begin melting the mini marshmallows and butter together in the microwave in 30-second increments or in a pot on the stove. Add the vanilla extract to the fully melted mixture and mix until well incorporated.

Add the rice Krispies and fold just until completely coated. Add in an extra handful of mini marshmallows, if desired. Press into a baking pan that has been well-greased with butter. Refrigerate while you make the ganache.

Heat the cream until scalding before adding the chocolate. Let sit for 1 minute before whisking until completely smooth. Spread the ganache over the rice krispie treats and then top with a sprinkle of flakey salt. Slice and serve immediately or refrigerate until ready to serve for the ganache to harden slightly.