Briefly mix 140 g butter and sugar with the whisk of the mixer. Knead the flour with your hands. Halve the dough. Roll out between 2 layers of baking paper approx. 2 mm thin. Place flat in the fridge with the paper for about 30 minutes.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Line two baking trays with baking paper. Take out 1 pastry sheet. Loosen the paper on both sides, then remove the top sheet.
Cut the dough into squares of about 3 cm. Distribute immediately on the trays with some distance between them. Bake one after the other in a hot oven for about 5 minutes. Remove from the oven and let it cool down for about 10 minutes. Then remove from the baking tray together with the paper and let it cool down.
Process the rest of the dough sheet in the same way.
Sift the icing sugar. Whip with 150 g butter and speculoos spice with the whisks of the mixer for about 7 minutes until creamy. Put into a freezer bag, cut off one corner. Spray a curl on the underside of half of the biscuits, cover with the remaining biscuits.
Press lightly, chill for about 30 minutes. Keep cool in a tin can for about 2 weeks.