1 tsp baking powder
120g/4½oz soft light brown sugar
250g/9oz icing sugar, sieved
140g/5oz self-raising flour
125g/4½oz speculoos biscuits (approximately 16 biscuits)
150²µ/5½´Ç³ú butter, at room temperature
4 free-range eggs
140g/5oz soft margarine (or butter, at room temperature), plus extra for greasing
3 tbsp Greek-style yoghurt