Stir in flour, coco powder, baking soda, and salt.
Spoon batter into well greased (or parchment lined) muffin tins. Add one square of bakers chocolate to each muffin, tucking it into the centre of the muffin, leaving a quarter or so of the square above the batter line.
Bake at 400 degrees for 15-20 minutes, until the muffins are firm and your kitchen smells like a chocolate factory.
Sprinkle a tiny pinch of kosher salt over the muffins tops, while the chocolate is still hot.