Preheat the oven to 180°C (356°F)
Melt the chocolate and let it cool off a little
Roughly chop the walnuts
Put flour, sugar, cocoa, cinammon, salt, baking powder and baking soda in a bowl and mix well
Add sparkling water, milk, oil and apple cider vinegar and mix well
Add chocolate and walnuts and mix shortly
Slice the plums and add 2/3 to the dough
Put the dough in a muffin tin and decorate it with the remaining plums, walnuts and chocolate drops
Bake for approx. 25-30 minutes – check with stick if well done