Preheat your oven to 350°F (175°C). Line your muffin tin with paper liners or grease it lightly with cooking spray.
In a large mixing bowl, beat together 1/2 cup of softened unsalted butter and 2/3 cup of granulated sugar until light and fluffy, about 2-3 minutes.
Add in 1 large egg and 1 1/2 teaspoons of vanilla extract. Mix until fully combined.
Stir in 1/2 cup of milk and 1/4 cup of plain Greek yogurt (or sour cream) until smooth.
In a separate bowl, whisk together 1 2/3 cups of all-purpose flour, 1 1/2 teaspoons of baking powder, 1/4 teaspoon of baking soda, 1/2 teaspoon of ground cinnamon, and 1/2 teaspoon of salt.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix; a few lumps are okay!
In a small bowl, mix together 3 tablespoons of melted butter, 1/3 cup of granulated sugar, and 2 teaspoons of ground cinnamon to prepare the cinnamon-sugar topping.
Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full. Then, drizzle the cinnamon-sugar mixture over the top of each muffin.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and let them cool in the tin for 5 minutes before transferring to a cooling rack. Enjoy warm or at room temperature!