Preheat your oven to 350°F (175°C). Spray a regular muffin tin with nonstick spray to prevent sticking. Make sure not to use muffin liners, as these muffins may cling to the liners.

In a large bowl, whisk together the eggs and packed brown sugar until well combined and slightly frothy. Next, stir in the unsweetened applesauce, nonfat plain Greek yogurt, and vanilla extract until the mixture is smooth.

Add the all-purpose flour, baking powder, and 1 teaspoon ground cinnamon to the wet mixture. Stir until just combined, being careful not to overmix. Then, gently fold in the peeled and diced apple, making sure the apple pieces are evenly distributed in the batter. I like to use a sweet apple variety for extra flavor, but any apple works well here.

Divide the batter evenly among the 10 prepared muffin cups. In a small bowl, mix the granulated sugar and the remaining teaspoon of cinnamon together. Sprinkle the cinnamon sugar mixture evenly over the tops of the muffins for a sweet, crunchy finish. This step is optional, but I think it adds a wonderful aroma and texture.

Bake the muffins in the preheated oven for 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Allow the muffins to cool in the pan for a few minutes before transferring them to a wire rack to cool completely. For best results, store muffins in the fridge, and if not consumed within 2-3 days, freeze them for longer shelf life.