Mix flour, salt, eggs and egg yolk, milk and mineral water with carbon dioxide to a tough dough. Leave to swell for about 30 minutes.
Press the dough in portions through a spaetzle press in plenty of boiling salt water. Let it simmer until the spaetzle rise to the top. Lift them out and let them cool down on an oiled baking tray.
Fry spaetzle in portions in hot butter for 3-4 minutes. Mix with roughly chopped hazelnut kernels.