Sift the icing sugar. Cut butter into pieces. Knead the flour, icing sugar, 2 egg yolks and butter with the dough hooks of the hand mixer. Finally, work into a smooth dough with your hands on a floured work surface.
Wrap the dough in foil and chill for about 30 minutes. Form approx. 20 walnut-sized balls from the dough and place them on a baking tray lined with baking paper. Press a depression in the middle of each ball with the handle of a wooden spoon.
Mix 1 egg yolk and milk. Spread the biscuits with the egg yolk milk. Bake the trays one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 8-10 minutes. Take out and let cool down.
Warm the jelly separately and stir until smooth. Fill half of the cookies with the quince jelly, the other half with the redcurrant jelly.