Melt the butter in a large, heavy saucepan over medium heat. Add marshmallows and stir until melted. Remove from heat and add cereal. Pour into a 13x9 cake pan that's been sprayed with non-stick cooking spray. Press down into pan and smooth the top with a spatula. Let it cool.
Cut into hearts with a cookie cutter.
Melt the chocolate and candy melts according to package instructions. Add a few teaspoons of oil to chocolate to make it easier for dipping. Add more, if needed. Add some oil to the candy melts too.
Dip rice krispy hearts in chocolate, then drizzle with candy melts. Or add sprinkles, if desired. Makes about 20 hearts.