Prep: Line a 9x9" pan with parchment paper on the bottom and all sides. Set aside.
Melt butter: In a large sauté pan or saucepan, melt the butter on low-medium heat.
Melt marshmallows: Once melted, add ¾ of the marshmallows and vanilla extract into the pan, keeping the heat at low-medium. Stir the marshmallows around until they melt into a puffy and gooey mixture.
Fold in cereal: Add all the Rice Krispies cereal and the remaining marshmallows. Fold them together until everything is coated. It's ready when the last marshmallows you added just begin to melt.
Pat into pan: Transfer the mixture to your lined pan. Gently pat it into the corners of the pan. Don't press down too hard otherwise, your rice krispie treats will be tough. Set aside to cool at room temperature.
Add hot cream to chocolate: Place your chopped chocolate into a small heat-safe bowl and set aside. Heat heavy cream on the stovetop or in the microwave until it just begins to simmer. Pour the hot heavy cream into the bowl of chocolate and let it sit undisturbed for 5 minutes.
Stir ganache: Stir the cream and chocolate mixture until all the chocolate has melted and it becomes a smooth and glossy ganache.
Spread ganache layer: Pour the ganache over the rice krispie treats and smooth it into an even layer.
Add toppings: While the ganache is still warm, sprinkle the top with toppings of your choice (eg. flaky salt, extra marshmallows, freeze-dried raspberries, sprinkles). Let the ganache set until firm to the touch before slicing into squares.