Once butter is melted, add vanilla and 6 cups of marshmallows. Melt the marshmallows slowly.
Right before the marshmallows are fully melted, add milk and the other 2 cups of marshmallows. Stir until the added marshmallows are starting to melt. (we don't want them fully melt so you will have a lumpy fluff)
Add the Rice Krispie Cereal. Stir until the cereal is fully coated with the marshmallow fluff.
Dump into an 9 x 13 greased dish. Use a greased spatula to spread out the treats and press down just a bit (you don't want them to so compacted that they become rock hard).
Allow the treats to fully cool and set up.
Using a heart shaped cookie cutter, cut out heart shaped treats and set aside.
Sprinkle a flat surface with powdered sugar.
After kneading the fondant, roll it out thinly on the powdered sugar surface.
Using the same cookie cutter, cut out a hear shape.
Spread a thin layer on top of the rice krispie treat.
Lay the fondant heart over the rice krispie treat and press down gently to put it in place.
Allow to harden a bit before writing on the hearts with the food writer. (I used a red one).