Heat the oven to 180 degrees Celsius. Line a 10-hole muffin tin with paper cases. Melt the chocolate in a food processor. In a very large bowl, whisk together the flour, sugar, cocoa, oil, 100 ml of sour cream, eggs, vanilla, and 100 ml of water. Beat with an electric whisk until smooth, then quickly fold in 100 g of chocolate pieces. Divide the mixture between the muffin holes, then bake for 20 minutes, until a skewer inserted comes out clean (make sure not to pierce a chocolate chunk). Allow to cool on a wire rack.
To make the frosting, put the remaining chocolate, the rest of the sour cream, and 3 tbsp of sugar in a small saucepan. Heat gently, stirring, until the chocolate has melted and the frosting is smooth. Allow to set in the fridge until thick enough to swirl over the muffins, then decorate.