Spray 9 x 13 pan, large mixing bowl, and spatula/spoon with cooking spray.
in saucepan on stove, or in a microwave safe dish, melt butter and marshmallows, stirring frequently until smooth.
Remove from heat, and add vanilla, if desired. Pour Rice Crispies into the large m ixing bowl, then the marshmallow mixgure. Stir until combined.
Butter hands and press into 9 x 13 pan. Cool in fridge for about an hour.
Place melting chocolate in a microwave safe bowl in 30 second increments until smooth. Do not over microwave.
Spread over top of rice crispie treats, then sprinkle on desired toppings. Cut squares, but leave in pan. Return to fridge to cool until toppings are set. Remove from pan and store in airtight containers for up to three days. Also freezes well.