First, make the cinnamon sugar. In a small bowl, whisk together the sugar and cinnamon for the swirl.
Prep the oven. Position a rack in the center of the oven and preheat the oven to 400°F.
Mix the dry ingredients for the muffin batter. In a medium bowl, whisk together the flour, sugar, cream of tartar, baking soda, nutmeg, and salt.
Mix the wet ingredients for the muffin batter. In a large bowl, whisk together the melted butter, milk, egg, and vanilla.
Mix the dry ingredients into the wet ingredients. Gradually whisk the dry ingredients into the wet ingredients until just combined. The batter will be fairly runny; use a rubber spatula to scrape down the bottom and sides of the bowl. Use the rubber spatula to finish mixing until smooth and well combined, 1 to 2 minutes more. Prep the muffin pans. Prepare two muffin tins by spraying the inside and border of every other cavity in the muffin tin generously with cooking spray.
Portion the muffin batter into the muffin pan, sprinkle with cinnamon sugar, and fill the empty cavities with water. Use a 1-Tablespoon or 3-Tablespoon cookie dough scoop to fill each sprayed cavity with 6 tablespoons of the batter. The first muffin tin will have 6 cavities filled, whereas the second muffin will will have 4 cavities filled.Sprinkle the top of each muffin, aiming for the batter and avoiding the pan, with 1 teaspoon of the cinnamon sugar.Pour water into the empty cavities of the muffin pan, filling each ⅔rds of the way full. Bake the muffins. Bake each muffin pan for 20 to 22 minutes, or until the muffins are domed and golden brown around the edges. A skewer inserted into the center of a muffin should come out with a few crumbs attached.
Cool the muffins. Cool the muffins completely to room temperature in their muffin pans on a wire rack.Because these muffins are so top-heavy, you'll run the risk of accidentally pulling the tops and bottoms apart if the cake is still warm! Wait until they are cooled completely before turning them out of the pan. Run an offset spatula underneath each muffin top once more and gently tilt the muffin upwards to turn it out of the pan. Serve and store. Serve at room temperature. The muffins are best on the day that they're made, but can be individually wrapped in plastic and stored at room temperature for up to 2 days.