Preheat the oven to 180° and grease the donut moulds. Mix the eggs and sugar and add the vanilla flavour. Melt the butter over medium heat and add to the mixture. Add flour, baking powder and salt (cocoa if desired) and yogurt and milk. Mix the mixture with a blender until combined. Pour the mixture into a piping bag and fill ¾ of each mould with the bag. Bake the donuts for 15 minutes at 160° C.