Preheat the oven to 375 degrees F. Spray a standard muffin pan generously with non-stick cooking spray. Set aside.
Add the butter and granulated sugar to a large mixing bowl or the bowl of a stand mixer. Cream together for about 2 minutes, until smooth and well combined.
Add egg. Add vanilla, milk and Greek yogurt and mix until combined.
Add flour, baking powder, baking soda, cinnamon and salt and fold in using a rubber spatula, just until combined. Don't overmix.
Spoon batter into prepared muffin tins, filling each about 2/3 full.
Bake in preheated oven for about 15-20 minutes or until a toothpick inserted in the center comes out clean (or with just a few moist crumbs).
Allow to cool in the muffin tin for a minute or two before removing to a wire cooling rack.
While the muffins bake, melt the butter for the topping.
In a separate small container mix together the granulated sugar and cinnamon.
Once the muffins are out of the oven and have cooled for just a few minutes, use a pastry brush to brush a thin layer of melted butter over each muffin.
Sprinkle generously with cinnamon sugar mixture. (You can also dip the muffin tops in the melted butter, followed by the sugar, but I've found that they look prettier when you use a pastry brush and then sprinkle the sugar.)