Mix the butter with the sugar, a pinch of salt and the egg yolk until frothy.
Rub the peel of two untreated oranges or lemons and squeeze out the juice. Stir a little grated zest and 2 tablespoons of juice into the foam mixture. Save the rest of the peel and juice for the icing. Then mix the flour with the baking powder and work under the mixture until a smooth dough is formed. Chill the dough for 1 hour.
Preheat the oven to 200 ° C top / bottom heat.
Roll out the cookie dough and cut out with a cookie cutter. Brush the edges with a little egg yolk and place the cookies on a baking sheet lined with baking paper. Bake for about 10 minutes until they are golden yellow.
For the icing, stir together the icing sugar, the juice and the hot water and brush the cookies with them while they are still warm. Decorate with grated peel and pistachio halves.