Line a slice tin with baking paper, leaving a little overhang to make the slice easier to remove.
Add the butter and sweetened condensed milk to a saucepan. Heat gently, stirring until the butter has melted and the mixture is smooth. Remove from the heat.
Place the crushed biscuits, dried cranberries and ½ cup white chocolate chips into a large bowl and mix to combine.
Pour over the warm butter and condensed milk mixture. Stir until everything is evenly coated and well combined.
Transfer the mixture to the prepared tin and press down firmly and evenly.
Place the remaining ¼ cup white chocolate chips into a heatproof bowl set over a saucepan of gently simmering water, making sure the bottom of the bowl doesn’t touch the water. Stir until melted and smooth.
Drizzle the melted white chocolate over the top of the slice.
Refrigerate for approximately 2 hours, or until firm.
Lift the slice from the tin using the baking paper, cut into pieces and serve.