Mix flour with baking powder. Pour onto the work surface. Press a depression in the middle. Add water, salt and sugar. Put butter or margarine in flakes on the flour rim. Quickly knead a smooth dough from the outside to the inside.
Place in a covered bowl in the refrigerator for 10 minutes.
Take the bowl with the dough out of the fridge and then form walnut-sized balls. Place on a baking tray well spread with margarine. Press a depression in the middle of each cookie with your thumb.
For the filling, chop the walnuts, mix with sugar and cinnamon. Fill into the cavities. Put the baking tray in the preheated oven on the middle shelf. Baking time: 20 minutes Electric oven: 200°C Gas oven: Level 4 or just under 1/2 large flame.
After 20 minutes baking time, remove the cookies from the oven. Place them on a cake wire.
For the icing, mix icing sugar in a small bowl with rum and hot water. Sprinkle the still warm cookies with a thin layer of icing sugar. Let them cool down. Makes 22 cookies.