Make the Muffins:

1. Preheat oven to 375°F (190°C). Grease a standard 12-cup muffin pan with non-stick spray.
2. In a large mixing bowl, cream together butter and sugar for about 2 minutes until smooth.
3. Add the egg and mix well. Then stir in vanilla, milk, and Greek yogurt until combined.
4. In a separate bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt.
5. Gently fold dry ingredients into wet ingredients until just combined. Do not overmix.
6. Divide batter evenly into muffin cups, filling each about 2/3 full.
7. Bake for 15-20 minutes, or until a toothpick inserted in the center comes out clean.
8. Let muffins cool in the pan for 1-2 minutes, then transfer to a wire rack.

Add the Cinnamon-Sugar Topping:

9. While muffins are still warm, melt butter for the topping. In a separate bowl, mix together sugar and cinnamon.
10. Using a pastry brush, lightly coat each muffin top with melted butter, then sprinkle generously with cinnamon sugar.
11. Alternatively, dip muffin tops directly into the melted butter, then into the cinnamon sugar for a thicker coating.