Put the sugar in a bowl. To it add egg and mix well with a fork, then add baking soda (observe the order in which the products are set to get soft chocolate muffins). Then add the oil and mix gently after it yoghurt and finally the flour with vanilla. Do not stir the mixture for a long time. Finally you can put a spoon cocoa to stain the muffins. Moulds for muffins prepared with paper shapes, are then filled with a mixture to the middle. On each muffin put broken pieces of chocolate in baking chocolate sinking into the dough. Bake at 170C degrees in preheated oven about 15 minutes. You can check whether they are prepared, as being stuck in one of those wooden skewer and if it does not sticky dough, then your chocolate muffins are ready. If you wish, you can sprinkle chocolate muffins with powdered sugar while warm. * Sometimes when I want to do different these delicious chocolate muffins, prepare sugar syrup to which add different flavors, depending on who will consume them. If only adults, and add some alcohol. In this case, the muffins should be well cooled down, and the syrup is hot. Dip the brush into the hot syrup and spread evenly over the cooled chocolate muffins, being careful not to excessively with the syrup.