Preheat the oven to 160c Fan/180c conventional.355F and line a 20 cm square tin with non-stick baking paper.

In a medium-sized saucepan set over a low heat, melt the butter and sugar together, stirring occasionally until the sugar has dissolved and the butter has melted. Remove from the heat, pour into a large mixing bowl and leave to cool slightly for about 10 minutes (don’t worry if the butter and sugar look like they have separated - this is normal).

Use a whisk (or an electric whisk) to bring the butter and sugar back together until smooth.

Gradually add the eggs, one at a time, whisking between each addition. Add the vanilla and whisk again. 

In a separate smaller bowl, sieve together the flour and baking powder. Add a third of the dry ingredients into the wet, then gently fold together. Repeat until all the dry ingredients have been added and you have a smooth mixture - take care not to overmix.

Roughly chop 75g of the Biscoff biscuits, saving the rest for decoration. Fold through the chopped biscuits along with the chopped dark and white chocolate until evenly distributed throughout the mixture. 

Pour into the prepared tin, then place the remaining Biscoff biscuits on top and sprinkle with the salt. 

Bake for about 30-35 minutes, the blondies should have a nice shiny crust on top but still feel soft to touch - take care not to over cook because you want the blondies to have a nice fudgy texture. Leave to cool completely in the tin before slicing into 16 small or 9 large squares.