Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.

In a large bowl, whisk together the protein powder, almond flour, baking powder, cinnamon, and salt until well combined.

Add the almond milk, egg, Greek yogurt, vanilla extract, and optional sweetener to the dry ingredients. Whisk until just combined with some small lumps remaining.

In a small bowl, mix together the coconut sugar and cinnamon for the swirl mixture.

Spoon half of the muffin batter into the prepared liners. Sprinkle half of the cinnamon-sugar mixture over the batter. Top with the remaining batter and finish with the rest of the cinnamon-sugar mixture. Use a toothpick or knife to gently swirl the layers together.

Bake for 18-22 minutes, or until the tops spring back when lightly touched and a toothpick inserted into the center comes out clean.

Let the muffins cool in the tin for 10 minutes before transferring to a wire rack to cool completely.

While the muffins are cooling, prepare the glaze by whisking together the protein powder and almond milk until smooth. Drizzle the glaze over the completely cooled muffins.