Prepare the Oven and Muffin Pan

Preheat the oven to 375°F (190°C) and lightly grease a standard muffin tin with non-stick cooking spray. Set aside.

Mix the Wet Ingredients

In a large mixing bowl, or using a stand mixer, beat the softened butter and granulated sugar together until smooth and well combined, approximately 2 minutes. Add the egg, followed by the vanilla extract, milk, and Greek yogurt. Mix until everything is incorporated.

Combine the Dry Ingredients

In a separate bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Ensure all dry ingredients are evenly distributed.

Combine Wet and Dry Ingredients

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Be careful not to overmix, as this can result in dense muffins.

Fill Muffin Cups and Bake

Divide the muffin batter evenly among the prepared muffin cups, filling each about two-thirds full. Bake in the preheated oven for 15-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs. Allow the muffins to cool in the pan for 1-2 minutes before transferring them to a wire rack to cool completely.

Prepare the Cinnamon-Sugar Topping

While the muffins are baking, melt the butter for the topping. In a small bowl, mix together the granulated sugar and cinnamon.

Finish the Muffins

Once the muffins have cooled slightly, brush each with a thin layer of melted butter, then generously sprinkle the cinnamon-sugar mixture on top. Alternatively, you can dip the muffin tops into the melted butter and then into the cinnamon-sugar mixture for an even coating.