Preheat the oven to 200 C / Gas 6. Line 12 muffin cups with cases.
In a small saucepan over low heat, melt the plain chocolate together with the butter. Let stand until cooled, about 10 minutes.
In a small bowl, stir together the chocolate-butter mixture with the buttermilk, sugar, egg and vanilla, until blended well.
In a large bowl, stir together flour, bicarb and salt. Make a well in the centre of the dry ingredients, pour in the chocolate mixture, and stir until just combined (don't over-mix). Stir in the chocolate chips. Spoon the muffin mixture into the muffin cases.
Bake at 200 C / Gas 6 for 20 to 25 minutes or until a skewer inserted in the centre of a muffin comes out clean. Remove muffin tin from oven and let stand at least 5 minutes, before removing the muffins and letting them cool on a wire rack. Serve warm or cooled; can be frozen as well.