Preheat the oven to 425°F / 218°C and line a muffin pan with 6 paper liners. Set aside.
In a medium-sized bowl whisk together the all-purpose flour, granulated sugar, baking powder, and salt.
In a separate bowl whisk together the buttermilk, sour cream, oil, egg and vanilla extract until combined.
Fold the dry ingredients into the wet ingredients with a rubber spatula, and mix until just combined (DO NOT OVER-MIX YOUR BATTER!). Your batter should not be all the way smooth, and it's ok if there are still some small chunks in it.
Gently mix the flour-tossed bits of chocolate into the batter until they're evenly distributed.
Divide the muffin batter evenly between the 6 muffin cups, filling them to the top of the liner.
If desired, add a sprinkle of coarse sugar and a few extra pieces of chocolate on the top of each muffin.
Bake the muffins for 6 minutes at 425°F/ 218°C to give the muffins a nice rise and a domed, golden brown top, then (keeping the muffins in the oven) reduce the oven temperature to 350°F / 175°C. Continue to bake for an additional 22-25 minutes.
Let the muffins sit in the pan for 10 minutes to cool, then move them to a wire rack to finish cooling.