Preheat oven to 375 degrees. Line a cupcake tin with paper liners or grease the tins.
Place the 3 Tablespoons cocoa and one Tablespoon butter in a small saucepan. (You can also use the microwave to heat the two together). Cook under low heat until butter melts, stir the melted butter into the cocoa keep cooking and stirring until well combined and smooth. Set aside.
In a medium bowl combine the flour,baking soda and salt, stir to combine, set aside.
Using a large bowl, beat the butter until creamy, gradually add the brown sugar to the bowl, beating until well incorporated. Add the egg,vanilla and melted chocolate to the bowl. Beat until well combined.
Add the flour mixture to the bowl in 3 additions, alternating with the buttermilk. Example...(Flour,buttermilk,flour,buttermilk,flour). Beat until just incorporated after each addition.
Fill muffin tins 3/4 full with batter. Sprinkle topping over each muffin. Bake at 375 degrees for 20 - 25 minutes or till tests done with a toothpick.