Line a 19 x 29 cm slice tin with baking paper.
In a microwave-safe bowl, melt dark chocolate, condensed milk, and butter in 30-second intervals, stirring between each until smooth.
In a large bowl, combine crushed biscuits and cornflakes. Pour the melted chocolate mixture over and mix well.
Press the mixture firmly into the prepared tin and refrigerate while preparing the topping.
Melt the white chocolate with vegetable oil, stirring until smooth. Pour over the chilled base and spread evenly.
Sprinkle chopped Caramello or Creme Eggs, mini chocolate eggs, Smarties, and sprinkles on top. Gently press them into the white chocolate.
Refrigerate for at least 4 hours or until fully set. Remove from the fridge 20 minutes before cutting into slices for easier serving.