Preparing chocolate rhubarb muffins

For the sprinkles, stir egg yolk, butter and sugar until slightly foamy, add the flour and form into a dough. Place it in the freezer and as soon as it is hard, use a grater to produce sprinkles. Prepare any kind of muffin tray. Melt the chocolate in a water bath, add the butter, stir, let it cool down to room temperature. Stir eggs, sugar and salt until slightly foamy, mix with the chocolate butter mixture. Sift flour, cocoa and baking powder together, stir them in without lumps. Afterwards, fold in the rhubarb pieces. Fill small muffin trays with the help of a piping bag. Evenly spread the sprinkles over the batter. Bake in the preheated oven at 210 °C for around 20 minutes, leave to cool.