Melt chocolate and butter in a saucepan and stir it into a homogenous mixture. Let the chocolate cool slightly. While the chocolate is cooling, whisk sugar and eggs until light and airy. Sift flour and cocoa powder into the egg mixture along with salt. Fold together gently until well combined. Carefully fold the cooled chocolate/butter into the egg mixture. Divide the chocolate cake batter into 12 muffin cups and bake in the middle of a preheated oven at 170 degrees Celsius (338 degrees Fahrenheit) C (conventional oven) for 25-30 minutes. Let the muffins cool on a wire rack.