Traditionally speculoos are made with 'cassonade' which is an unrefined dark moist sugar. The darkest sugar I had to hand was Billingtons molasses sugar which seemed to me to fit the bill nicely.
250g butter (at room temperature)
250g dark brown sugar
1 tsp baking powder
1 tsp ground mixed spice
1 tsp ground ginger
Beat the butter, sugar and spices together until they form a thick paste. Add the egg and beat again until combined. Then sift in the flour and baking powder a little at a time and mix until you have a soft dough.
Split the mixture in half, wrap each ball of dough in cling film and pop into the fridge. the original recipe says for 12 hours, but those were 12 hours I didn't have to spare, so I just left mine for an hour or so.
Roll the dough out until it is about 3mm-4mm thick. As the dough is sticky (and to avoid using more flour), this might best be done between baking parchment or cling film.
Cut into rectangles or festive shapes and bake in an oven preheated to 180C for around 10 minutes. Leave to cool and then enjoy (with or without children!).