In a large pot, over low heat melt butter. Add marshmallows. Stir occasionally until gooey. This takes a little time as you need to keep your heat low or the marshmallows can burn and turn brown.
Remove from heat and stir in vanilla (eyeball it...a little more doesn't hurt).
Pour in rice krispies and stir until combined.
Spray a 9x13 inch pan with cooking spray.
Pour rice krispies into pan. With buttered hands or a spatula press down gentley until even. Set aside to cool.
Pour chocolate into microwave safe bowl and microwave 40-60 seconds or until softened, stirring at 20 second increments.
Add a couple drops of food colouring into white chocolate, stirring after each addition until you reach your desired colour.
Pour melted chocolate over rice krispies, spreading for even consistency.
Top with your favourite ghoulish sprinkles.
Allow to cool in refrigerator for 30 minutes for chocolate to set up.