Prep the pan and oven. Line a 12-cup muffin tin with paper liners and preheat the oven to 375 F.
Mix the dry ingredients. Combine the flour, 2 1/2 teaspoons of cinnamon, baking powder, cream of tartar, baking soda, and salt in a bowl; whisk to blend thoroughly.
Make the batter. In a mixing bowl, combine the 3/4 cup of sugar with the eggs, milk, sour cream, vanilla, and melted butter. Whisk until smooth. Stir the dry ingredients into the wet mixture and stir until just combined. Avoid overmixing.
Fill the pan. Fill the muffin cups about 2/3 full.
Make the topping. Combine 3 tablespoons of sugar with 1 1/2 teaspoons of cinnamon. Sprinkle over the muffins.
Bake the muffins. Transfer the pan to the oven and bake the muffins for 17 to 20 minutes or until a toothpick inserted into the center of a muffin comes out clean or with just a few crumbs clinging.