Put butter in a saucepan over medium low heat and let it melt.
Mix quickly to combine, and shape the mixture as desired. You can put few tablespoons of Rice Krispie mixture in a heart shaped cookie cutter (for example), placed on a parchment paper.
After 2 minute or less, pass a knife at the edges of the cookie cutter and press down on the rice krispie treat with hands to take it out of the cookies cutter.
You can sprinkle the Rice Krispies treats with decorating sugar, sprinkle mix, etc.
Let set at room temperature for at least 30 minutes.