Hi @bizi. Yes, a thin sharp knife is needed, so I even sharpened one right before. However, the recipe also calls for soaking the hazelnuts in water for two hours before incorporating into the dough. This softens them a bit, facilitating the cutting within the dough. It was no problem. These were comparatively easy to make.

Not as fancy looking as others I've made, but especially delicious with Earl Grey tea with milk and sugar. Pictured. Some with cocoa, some without.