Beat butter and sugar until frothy, add egg yolk and butter vanilla flavor and stir again well. Slowly stir in the flour and salt. Finally stir in the chocolate.
Mix the sugar and cinnamon well. Shape the dough into small balls, roll evenly in the sugar-cinnamon mixture and spread on a baking tray lined with baking paper.
Bake in the preheated oven for 12 minutes at 190 ° C fan oven.