Preheat the oven. Set your oven to 350°F. Line a muffin tin with paper liners or spray it well with cooking spray.
Mix the wet ingredients.In a large mixing bowl, combine the butter, sugar, milk, and Greek yogurt. Stir until smooth and creamy.The Greek yogurt is very forgiving — I never measure exactly, and they always turn out great!
Mix the dry ingredients.In a separate bowl, whisk together the flour, cinnamon, baking powder, salt, and nutmeg.
Combine wet and dry.Slowly add the dry ingredients to the wet ingredients, stirring until just combined. Don’t overmix — stop when you don’t see dry flour anymore.The batter will be sticky! Spraying your scooper or spoon with cooking spray really helps. I use a scoop with a lever handle that pops the batter out, but the spray trick makes it even easier.
Fill the muffin cups.Scoop the batter evenly into the muffin tin. This recipe makes about 11–12 muffins.If your family loves them as much as mine does, you might want to double the batch!
Bake. Bake for about 25 minutes, until the tops are lightly golden and spring back when you touch them. You can also check with a toothpick — it should come out clean. Every oven is a little different.
Cool and make the topping. Let the muffins cool in the pan for a few minutes. While they’re cooling, melt the butter in a small bowl and mix together the sugar and cinnamon in another.
Add the topping. Brush the tops of the warm muffins with melted butter and sprinkle generously with cinnamon sugar.