Preheat the oven to 200C/180C Fan/Gas Mark 6. Line a 9x13" pan with greaseproof paper (HACK: use paint-free bulldog clips to keep the baking paper attached to the tray).
In a large bowl, or the bowl of a stand mixer, cream the butter and brown sugar for about 3 minutes on a high speed (with an electric mixer). Add the eggs and vanilla and mix again.
Add in the oats, biscuits, flour, baking powder and salt, and fold everything together into a sort of cookie dough/paste. Set aside.
Make the fudge by combining the condensed milk with the broken up white chocolate in a medium saucepan. Set over a low heat and stir until the chocolate has melted. Turn off the heat and add the Biscoff spread. Stir together until just combined.
Pour about 2/3 of the oat mixture into the baking tin and spread out (this may take some effort, press into the edges with a wooden spoon). Pour over the fudge. Then 'sprinkle' over the remaining oaty mixture, it doesn't have to completely cover it (refer to video).
Bake for 20-25 minutes, until the top is a golden brown (and the kitchen smells amazing). Remove and let cool on a wire rack.
Optional: Decorate by drizzling over 1 tbsp melted Biscoff, and sprinkling over some crushed biscuits. Cut with a sharp knife and serve!