Preheat oven to 180°C (350°F). Lightly grease a muffin tray for 6 large or 12 small muffins.
Combine the flour, baking soda, baking powder, salt and cinnamon in a mixing bowl and set aside.
Combine the milk and yogurt in a small jug and set aside.
Using a mixer, cream the butter and sugar until light and fluffy. Add the egg and beat until just combined.
Add the flour mixture in thirds, alternating with the milk mixture and mix until just combined – do not over mix. Spoon the batter into the prepared tin, filling approximately 2/3 full.
Bake for 25 minutes until golden brown and firm to touch.
For the coating:
Melt the butter and place in a bowl. In a separate bowl, combine the sugar and cinnamon.
Once the muffins have cooled completely, dunk first in the melted butter and then coat with the cinnamon sugar.