Preheat the oven to 350. Line a muffin pan with paper liners. In a medium saucepan over a medium heat, melt butter and chocolate until smooth, stirring frequently. Remove from the heat and allow to cool slightly.
Stir in the sugar, egg, buttermilk and vanilla extract. Gently fold in the flour and baking soda until just blended. ( do not overblend even though the mixture looks lumpy ) Fold in the blueberries.
Spoon the batter into prepared pan, filling 2/3's of the cup with batter. Baking for 25-30, until center tests done with toothpick. Remove from oven and place muffins on a cooling rack to cool. (they will get soggy if left in the pan). To decorate, drizzle with the melted chocolate and serve warm or cool.