Add the yogurt, milk, eggs, and melted butter to a medium bowl. Whisk to combine into a smooth mixture.
Add the flour, sugar, baking powder, and salt. Whisk gently to combine.
Remove 1 cup of batter and put it in a small bowl. Add the cinnamon and stir to combine. Add to the zip-top plastic bag. Seal the bag.
Heat a large nonstick skillet over medium-low heat. Melt a little butter and swirl to coat the pan.
Spoon out about 1/4 cup batter per pancake. Snip 1 bottom corner off the cinnamon-batter bag and pipe a swirl onto each pancake. (You'll want to do this step quickly as the batter is runny.)
Cover and cook for 4 minutes, or until you see bubbles on the top and the pancake is set in the middle. (Do not flip over or you won't be able to see the swirls!)
Stir together the Easy Icing, if using.
Serve warm with syrup or drizzle with Easy Icing.
Notes
To make this as a Sheet Pan Pancake: Preheat oven to 400 degrees F. Line a quarter sheet pan with parchment paper and grease the sides with nonstick spray. (You can also use a half sheet pan, just spread it out to about 1/2 inch thick. It's OK if it doesn't go to the edges.) Pour out batter and spread gently with a spatula. Make a bunch of swirls on top with the cinnamon batter. Bake for about 12 minutes or until just set.
Store for 3-5 days in the fridge in an airtight container. Warm for 15-30 seconds on a heat-safe plate in the microwave. Or freeze between layers of parchment paper (or in a single layer) in a zip-top bag.
To use whole wheat flour, you may need 1-2 additional tablespoons milk since it absorbs liquids more readily. (The swirls will be less obvious since the batter will be browner.)
Gluten-free: Use a 1:1 style of gluten-free flour.
Dairy-free: Use nondairy Greek-style yogurt and nondairy milk.
Egg-free: Use my Fluffy Pancakes without Eggs recipe and modify to make part of the batter cinnamon.