In a large mixing bowl, combine the crushed digestive biscuits and melted butter. Mix until the biscuits are fully coated.
Add the mini marshmallows, vanilla extract, and a pinch of salt to the biscuit mixture. Stir gently until well combined.
Line a square baking dish (approximately 8×8 inches) with parchment paper, leaving some overhang for easy removal.
Transfer the mixture into the prepared baking dish and press it down firmly with the back of a spoon or your hands to create an even layer.
Melt the milk chocolate in a microwave-safe bowl in 30-second intervals, stirring in between until smooth. Pour the melted chocolate over the biscuit and marshmallow layer, spreading it evenly.
Melt the white chocolate in a similar manner and drizzle it over the milk chocolate layer for decoration.
Refrigerate the slice for at least 30 minutes or until set.
Once set, lift the slice out of the baking dish using the parchment paper overhang. Cut into squares and serve.