Grate the zest of one of the mandarin oranges. Squeeze the juice from all 4 mandarin oranges. Measure 125 ml of the juice into a pan with the zest, jam sugar and lemon juice. Bring to a rolling boil and boil for 4 minutes, stirring constantly. Pour the jelly into a bowl or jam jar and cool in the fridge.
Cream the butter and icing sugar. Beat in the egg yolk, spekulatius spices and salt. Heat the cream to hand temperature and mix gradually into the buttercream. Mix in the flour.
Heat the oven to 200 °C. Fill the biscuit dough into a piping bag with a small round nozzle and pipe 50 to 60 spirals onto a baking sheet lined with baking paper. Bake in the oven for 15 mins or until golden. Remove from the oven and cool on wire racks.
Spread half of the biscuits with a little mandarin jelly and sandwich with a second biscuit. Dust lightly with the spiced icing sugar.