Preheat the oven to 150°C. Line 2 baking trays with greaseproof paper and set aside. Spread the flaked almonds out into an even layer on a plate and set aside, ready for rolling the cookies in later.
Place the grated marzipan, ground almonds, icing sugar and egg whites into a large bowl and mix with a wooden spoon until the mixture begins to clump together. Use your hands to knead the mixture, so that it forms a smooth, sticky dough.
Take a small amount of dough (about 25g) and roll it into a ball (see Note 1). Roll the ball between the palms of your hands to form a log, about 9cm long. Roll the log in the flaked almonds to coat it, then bend it into a crescent shape and place it onto one of the lined baking trays. Repeat this process for the rest of the dough.
Once you have all of your mandelhörnchen rolled, bake one tray at a time in the preheated oven for 10-12 minutes, or until the cookies are golden. Allow the cookies to cool completely on the tray before dipping (if you choose to do so)/serving.
To dip the cookies, melt the chocolate in the microwave (see Note 2, or use your preferred method for melting). When the chocolate is melted, dip the end of each crescent into the chocolate, allowing any excess to drip off, back into the bowl, before placing the cookies back on the baking paper to set.
Once the chocolate has set, you can transfer the biscuits to an airtight container- they will keep for about 3 weeks at room temperature.