1. Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or lightly grease each cup.

2. In a large mixing bowl, whisk together the almond flour, protein powder, granulated erythritol, baking powder, and salt. Make sure there are no lumps and the ingredients are evenly distributed.

3. In a separate medium bowl, whisk together the Greek yogurt, eggs, unsweetened applesauce, melted coconut oil (or butter), and vanilla extract until smooth and well combined.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spatula until just combined. Be careful not to overmix, as this can lead to tough muffins. A few small lumps are perfectly fine.

5. Gently fold in the fresh or frozen blueberries. If using frozen blueberries, do not thaw them first; add them directly to the batter to prevent them from bleeding too much color into the muffins.

6. Divide the batter evenly among the 12 prepared muffin cups. Each cup should be about two-thirds full.

7. Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the tops are golden brown and a toothpick inserted into the center of a muffin comes out clean.

8. Remove the muffins from the oven and let them cool in the muffin tin for 5-10 minutes before transferring them to a wire rack to cool completely. This helps prevent them from becoming soggy.