With the rack in the middle position, preheat the oven to 350°F (180°C). Line a 12-cup muffin pan with paper or silicone cups.
In a bowl in the microwave oven or over a double boiler, melt the chocolate and butter. Add the brown sugar and combine with a whisk for 1 minute. Add the eggs and whip for 1 more minute. Add the flour and combine for 1 minute.
Divide the batter among the muffin cups. Place 2 pieces of candy bar in the middle of each cup, and push them slightly into the batter.
Bake for 14 minutes, or until a toothpick inserted in the centre of the brownie (but not through the Mars bar) comes out with a few wet crumbs (not completely clean).
Let the brownies cool for about 2 hours before eating them.