Delicious wee snack, perfect post-training (but don't be tempted to go overboard). I didn't have mini-muffin paper cases so made a dozen full sized muffins).
Preheat oven to 200℃/390°F. Line 36 mini-muffin holes with paper cases (or make 3 batches with a 12-hole tin.) Combine flour, cocoa and sugar in a large bowl. Add egg, yogurt, banana and oil. Stir until just combined.
Divide mixture among apper cases. Press a chocolate button half into each muffin. Bake for 15 minutes, or until an inserted skewer comes out clean. Transfer to a wire rack to cool.
Variations
Using macadamia oil will add a richer, slightly nutty flavor.