Line an 8x8-inch tin with baking paper and set aside. Crush the Bourbon biscuits into fine crumbs and place them in a large bowl. Add the melted butter, condensed milk and crushed Maltesers, then mix until the mixture comes together like a soft, sticky dough. Press the mixture firmly and evenly into your prepared tin.
In a microwave-safe bowl, add the milk chocolate pieces along with the remaining condensed milk. Melt in short 20–30 second bursts, stirring each time, until smooth, thick and glossy. Pour the chocolate topping over the biscuit base and spread it out evenly to the edges. Decorate the top with festive Maltesers.
Chill the traybake in the fridge for around 3 hours or until completely set. Once firm, slice into squares and enjoy your festive Malteser tray bake!
