Preheat oven to 200 degrees and place paper cupcake cases into tins.
Chop the Tim Tams and Chocolate into small pieces and set aside
Sift the flour, cocoa powder and bicarbonate of soda into a large bowl. Add the sugar, Tim Tams and Chocolate and make a well in the centre.
In a jug, whisk together the buttermilk and eggs, and pour into the well. Add the butter and fold mixture gently together with a metal spoon until just combined.
Spoon mixture into muffin cases (makes around 18 muffins) until around 3/4 full. Bake for approx 20 minutes until muffins have risen and are cooked through.
Let the muffins cool in trays for 5 minutes before moving to a wire rack to cool completely.