Preheat the oven to 180°C, fan 160°C, gas 4. Line a bun tin with fairy cake cases. Put the 100g of chocolate pieces, butter and 50ml water into a small pan. Stir over a gentle heat until the chocolate melts, then set aside.
In a large bowl, mix together the flour, sugar and cocoa. Break up any lumps of sugar with your fingers. In another bowl, beat together the egg and yogurt.
Stir the egg and chocolate mixtures into the flour and mix well. Spoon this mixture into 15 fairy cake cases (you may have to cook these in a couple of batches). Bake for 15-20 minutes until the cupcakes have risen and are firm to the touch. Transfer to a wire rack and allow to cool completely.
For the icing, heat the cream in a pan until starting to boil. Stir in the 50g of finely chopped chocolate and sugar until the mixture is smooth. Allow to cool, then spoon over the cupcakes.
Melt the white chocolate, then drizzle spiral shapes over the cupcakes, or use the icing pen. With a cocktail stick, pull lines through the circles to make the web effect. Allow to set.