Place the butter in a bowl, add the sugar, vanilla sugar and salt. Stir in the egg, beat for approx. 3 mins. using the whisk on a hand mixer. Mix the almonds, flour, baking powder and gingerbread spice, stir into the mixture. Shape approx. 40 balls by hand, place on two baking trays lined with baking paper. Top with the kumquats and press down gently.
Approx. 10 mins. in an oven preheated to 180 °C (convection). Remove from the oven, leave the biscuits to cool on a rack. Mix the icing sugar and lemon juice, drizzle the icing over the biscuits.