Preheat the oven to 180 ° (160 ° convection). Bring the cornstarch with baking cocoa, sugar and milk to the boil in a large saucepan while stirring. Add the butter and chocolate in pieces and stir until both have melted. Remove the pot from the oven. Add the oil, eggs and powdered sugar and mix into a smooth batter. Sift flour, baking powder and salt over it and fold in. Grease the wells of the muffin tray and dust with a little flour. Pour in the batter. Bake in the oven for 20-25 minutes. Take out, let cool down briefly, then remove the muffins and let cool down. Pour the vanilla sauce over the muffins to serve.