Start this recipe by melting together the chocolate, butter substitute and milk. Remove off the heat. Sift together the flour and bicarbonate of soda and put aside. In another bowl, mix together the egg, sugar and natural yogurt. Add the flour mixture, along with the chocolate mixture and fold together until just combined. Line a muffin tin with 10 paper cases and divide the muffins batter equally. Bake in a preheated oven at 190`c for approximately 20 minutes. When cooked, remove from the oven and allow to cool completely on a wire rack. When ready to decorate, place the icing sugar in a bowl and mix together with a little warm water, only a teaspoonful at a time until you obtain a thick, glossy icing that holds it shape. Use a small palette knife or the back of a spoon to quickly glaze the top of the muffins. Next, place the melted chocolate in a small piping bag and pipe a spiral on top. Using a small skewer or cocktail stick, drag the chocolate from the center of the muffins outwards, at 5 or 6 intervals creating a spider`s web. Allow the icing to set for an hour before serving.