Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners.
In a large mixing bowl, whisk together the whole wheat pastry flour, protein powder, baking powder, baking soda, ground cinnamon, and salt. Make sure there are no clumps of protein powder.
In a separate medium mixing bowl, whisk together the eggs, unsweetened almond milk, unsweetened apple sauce, maple syrup (or honey), melted coconut oil (or olive oil), and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients. Stir gently with a whisk or spatula until just combined. Be careful not to overmix the batter – a few small lumps are perfectly fine and actually help keep the muffins tender! Overmixing can lead to tough muffins.
In a small bowl, mix together the brown sugar (or erythritol) and 1 tablespoon of ground cinnamon.
Divide about half of the muffin batter evenly among the 12 prepared muffin cups.
Sprinkle about half of the cinnamon swirl mixture evenly over the batter in each cup. Then, top with the remaining muffin batter, filling each cup about two-thirds full. Sprinkle the remaining cinnamon swirl mixture over the top of the batter in each cup.
Using a small knife or a toothpick, gently swirl the cinnamon mixture into the top layer of the muffin batter a few times. Don't overdo it, or the swirl will disappear.
Bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean. The tops should be golden brown and spring back when lightly touched.
Let the muffins cool in the muffin tin for 5-10 minutes before transferring them to a wire rack to cool completely.
While muffins cool, whisk all glaze ingredients together in a small bowl until smooth and desired consistency is reached. Drizzle over cooled muffins.