To prepare the muffin batter, sift the flour, cocoa powder, salt, baking powder and baking soda into a bowl. Chop the chocolate and add to the flour mixture. Set aside.
In a separate bowl, beat the oil and the sugar. Add the eggs, one at a time, beating between each addition. Add the vanilla and the coffee powder. Beat until smooth. Finally, add in the milk and vinegar. Stir in the flour mixture until fully incorporated.
Spoon the batter into greased muffin tins. Place one or two frozen cranberries into the center of each of the muffins. Bake in a 350°F/176°C oven for 18 to 25 minutes, or until a cake tester inserted into the center comes out clean.