First beat the egg with the sugar, then add baking soda to yogurt and a drop of vinegar (if milk is not sour), stir and add to mixture. Then add the oil, cinnamon, cocoa, essence and finally the flour to a thick mixture (but can be poured), add the walnuts and distribute the forms in which we previously put oiled paper shapes. Once spread the mixture, add in a little piece of chocolate into each muffin, push it to cover the mixture, but does not touch the bottom. Bake at 230C degrees for silicon shape placed in another tray bake on the bottom of the oven and a conventional metallic form in the middle, check with a stick, bake 15-20 minutes. * The mixture is approximately 24 muffins.