Cut a 15 cm (6 inch) square from baking paper and use it as a template to cut 7 more. Fold them all into quarters. Open out flat and set aside.
Sift the flour, cocoa powder and baking powder into a bowl and stir in the sugar. Beat together the egg, milk and melted butter in a small bowl, then mix into the dry ingredients until just combined (don't overmix). Fold in the chocolate and pecan nuts.
Spray each square of baking paper with spray oil and press each piece into the hole of a muffin tray. Spoon the chocolate mixture into the lined holes and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes until risen and golden. Leave to cool slightly on a wire rack and serve warm.