Spray an 8×8-inch / 20×20-centimeter square baking tin with cooking spray, line with parchment, and set aside until ready to use.

Crisp Layer:

In a large pan, combine golden syrup and butter. Bring mixture to a boil over medium-high heat, then remove pan from heat and stir in the vanilla extract.

Add Rice Krispies into the syrup mixture, stirring until well-combined. Pour into prepared tin.

Using an oiled spatula, gently press mixture into the tin, creating a smooth, even cereal layer. Set aside while you prepare the rocky road.

Rocky Road Layer:

In a large pan, melt chopped chocolate and peanut butter over low heat, stirring gently to combine.

Stir in the peanuts, followed by the marshmallows, and half of the mini M&Ms. Pour rocky road over the crisp layer, making sure it is evenly distributed and completely covering the crisp layer. If necessary, use a spatula or spoon to push rocky road mixture into the corners of the tin.

Sprinkle rocky road with remaining mini M&Ms.

Decoration:

In the microwave or over a bain marie, melt 2-3 tablespoons of chopped milk chocolate. Transfer melted chocolate to a piping bag and pipe evenly over the top of the rocky road. Sprinkle with rainbow nonpareils.

Place tin in the refrigerator and chill until firm, then slice into squares.

Store rainbow rocky road bars in an airtight container at room temperature for up to five days. If your kitchen is very warm, you can also store your bars in the fridge.)