Preheat the oven to 302 °F / 150 °C (top/bottom heat).
Beat the leftover egg yolks with the powdered sugar, salt, and vanilla sugar using a hand mixer until foamy.
Fold in the rum and the grated walnuts (almonds or hazelnuts) into the foamy mixture.
Fill the mixture into a pastry (piping) bag and create tiny mounds onto the baking tray lined with baking paper.
Once a baking sheet is full, bake the cookies on the second rack from the bottom for about 20 minutes.
If you have opted for the variant with baking wafers, pipe the mass directly onto the wafers.
Add a walnut (almond or hazelnut) on top before baking.
Please note that all my recipes are developed and tested in metric grams. I recommend using digital scales for more accurate results. I provided a conversion to US customary in the recipe, but please note that I haven't tested it using this method.
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Keyword Christmas Cookies, cookie, cookie recipe, easy, Extra Egg Yolk, Extra Yolks, gluten-free, Leftover Egg Yolks