In a bowl Tap egg add the butter, the oil, yogurt and powdered sugar. Stir ingredients with mixer until well mixed. To this mixture add the starch and cocoa and mix again with the mixer. In a few tablespoons of flour stirring baking powder and vanilla and add to the bowl with hands begin to interfere dough as little less add flour until it forms thick dough. Kneading nice and let rest 15 minutes. Take a piece of dough as walnut, between palms shape the ball, which rolled through the palms again. Shaped cookies redim in pan on baking paper. Bake at 180 C for 10-15 minutes * The decoration is of 50 g dark and white chocolate melted in a water bath with 30 g butter. Let them cool completely, ordered them on baking paper to each other and decked them with a spoon. In the original recipe are unadorned.