Line a 12-hole mini muffin tin with paper mini muffin cases or lightly grease. Place the butter and peanut butter in a small saucepan over a low heat and warm until just melted. Stir gently, then set aside.
Meanwhile, sift the flour, baking powder and salt into a large bowl, then stir in the sugar. Whisk together the egg, buttermilk and melted butters in a jug. Pour into the dry ingredients and stir until just combined.
Spoon the mixture into the prepared muffin tin, then scatter over the chocolate chunks or chips.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 10–15 minutes until risen and golden. Serve warm.