1. Preheat the oven to 180°C (160°C fan) gas mark 4. Grease and line the base and sides of a loose bottomed 20 x 20cm (8 inch) baking tin. 
2. Place the butter and 50g of the white chocolate into a heatproof bowl set over a pan of barely simmering water. Gently stir until it melts then set aside to cool.
3. Add the melted chocolate mixture to a large mixing bowl with both sugars then use a hand held electric mixer to whisk this mixture together for 1-2 mins until slightly aerated, then add the eggs and vanilla and continue to whisk until well combined. Fold through the remaining 100g white chocolate chunks, flour and speculoos spice mix with a pinch of salt until evenly combined. 
4. Transfer this mixture to the lined tin and bake for 25-30 mins, until firm and light golden brown on top and just set. If you insert a skewer into the centre there should still be some moist crumbs clinging to it when cooked.
5. Leave to cool completely before cutting into squares and serving. 

Storage tip: Store in an airtight container at room temperature for up to 3 days.