Roast ground hazelnuts and walnuts in a dry pan until fragrant, stirring constantly. Remove from pan and let cool.
Preheat the oven to 140°C (approximately 300°F).
Beat egg whites and salt until stiff. Gradually add vanilla sugar and sugar. Fold in roasted nuts and gingerbread spices. Place mixture into a piping bag and pipe 5-6 cm (approximately 2 inch) long, narrow strips on a baking sheet lined with parchment paper. Bake for 15-20 minutes, until light golden brown. Remove from oven and let cool on the sheet.
Combine powdered sugar and lemon juice, whisk until thick. Pour into a fine-tipped piping bag and use to decorate cookies. Sprinkle with slivered hazelnuts and serve.