Combine all of the dry ingredients in a small bowl - flour, baking powder, baking soda, and ground cinnamon. Whisk to combine and set the bowl aside.
Combine the sugars and all of the liquids to a medium bowl - sugar, melted butter, milk, yogurt, vanilla extract, and apple cider vinegar. Whisk the ingredients together until well combined - about 30 seconds to 1 minute.
Add the dry ingredients to the wet ingredients, gently folding until no more flour can be seen. Cover the batter and let it rest on the countertop for 30 minutes.
While the batter is resting, peel, core and chop the apple(s) into ¼-inch pieces and roughly chop the walnuts. (one medium or 2 small apples should be enough)
Prep the 6-cup jumbo muffin pan by lining each cup with a large paper liner or make your own with parchment paper.
Preheat the oven to 425F. Add the chopped apple pieces (1 cup) and the roughly chopped walnuts (¾ cup) to the batter, gently folding them in until evenly distributed.
Divide the batter among the 6 jumbo cups - about a heaping ¼ cup of batter per cup. Place 3 pieces each of chopped apple and walnut to the top of each muffin.
Bake the muffins at 425F for 8 minutes. Decrease the oven temperature (do not open the oven door) to 375F and bake for another 20 to 25 minutes. The tops should be starting to brown and a toothpick inserted should come out clean.
Once out of the oven, let the muffins rest in the pan for 5 minutes and then transfer them to a cooling rack. The muffins can be served warm or at room temperature.
Prepare the cinnamon glaze by combining the powdered sugar, milk and cinnamon in a small bowl. Stir until well combined and smooth.
Drizzle the glaze over warm or room temperature muffins and serve.
Store the muffins at room temperature in an airtight container. The are best served the same day.