Place egg whites with sugar and hazelnuts in a saucepan and heat to about 50 °C while stirring. (The mixture should not stick to the bottom of the saucepan.) Mix in remaining ingredients and mix firmly to obtain a good binding. While still warm, pour into a piping bag fitted with a hole nozzle.
Place wafers on prepared baking sheet and pipe about 15 g of mixture onto each wafer. Place the piping bag opening in the center of the wafer and press the mixture from the inside out to the edge of the wafer. Do not spread too thinly so that the gingerbread is nice and moist. Press in a walnut half of each as a garnish.
Let dry at room temperature at least overnight or longer. It is best to do a baking test: If the mixture still runs off the wafer too much, continue to let dry.
To finish, bake in a preheated oven at 165-175 °C for 9 to 11 minutes until light brown.