Preheat your oven to 350°F and line a muffin tin with parchment liners or grease lightly with coconut oil.
In a medium mixing bowl, whisk together the Greek yogurt, egg, melted coconut oil, and vanilla extract until smooth.
Add the vanilla protein powder, oat flour, baking powder, half of the cinnamon, and sea salt to the wet ingredients, stirring until just combined.
Core and finely dice the apple, then gently fold the pieces into the muffin batter.
In a small separate bowl, combine the chopped pecans, maple syrup, and the remaining cinnamon to create the streusel topping.
Spoon the batter into the prepared muffin cups, filling each about three-quarters full.
Sprinkle the pecan streusel evenly over the top of each muffin.
Bake for 18 to 22 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.