Line an 8×8-inch square tin with parchment in a criss-cross pattern, allowing a 1-inch overhang on all sides. Spray generously with nonstick spray. Set aside.

Add the white chocolate and sweetened condensed milk to the top of a double boiler, or sit a large metal bowl over a stock pot of simmering water to create a bain-marie.

Melt chocolate, stirring periodically, until completely melted. Stir in the oil.

Weigh* 250 grams white chocolate into a smaller bowl. Tint with purple oil-based food coloring. Mixture will be sticky.

With a small stepped palette knife or flexible rubber spatula, spread purple chocolate across bottom of prepared pan. Freeze while you prepare the next layer.

Repeat the steps above, using blue, green, yellow, orange, and red food coloring.**

Loosely cover tin with aluminum foil or clingfilm/plastic wrap. Refrigerate 2 hours, or until set.

Lifting by the parchment, transfer fudge slab to a large cutting board.

With a sharp, hot*** chef’s knife, trim ends then slice into 6 x 6 rows.

Store rainbow fudge in an airtight container at room temperature for up to 1 week.