Line an 8x8-inch tin with baking paper.
Place 8 Bourbon biscuits onto a baking tray. Warm them up in the oven for 3 minutes at 180°C so you can easily slide them apart, making 16 halves. Set aside.
Crush the remaining 450g of Bourbon biscuits into fine crumbs.
Place the crumbs into a large bowl with the melted butter and 120ml of the condensed milk (save the rest of the tin for step 6).
Mix well to make a wet dough-like mixture then press it into your lined tin.
Break the milk chocolate into pieces and place it into a saucepan with the rest of the condensed milk. Put the pan over low heat and stir gently until the chocolate has melted. You should have thick and glossy mixture.
Spoon the topping over the biscuit base in the tin and spread it out to the edges. Top with the Bourbon biscuit halves, placing them in 3 rows of 5.
Put your Bourbon tray bake in the fridge for 3 hours to set then cut into slices and enjoy!