Prep the pan: Preheat your oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or grease well.
Mix dry ingredients: In a large bowl, whisk together almond flour, coconut flour, cinnamon, apple pie spice, baking powder, baking soda, salt, and sweetener until no lumps remain.
Whisk wet ingredients: In a separate bowl, beat the eggs, almond milk, melted butter, vanilla, and apple extract (if using) until smooth and slightly frothy.
Combine: Pour the wet mixture into the dry and stir with a spatula until just combined. The batter will be thick but scoopable. Let it rest for 3–4 minutes so the coconut flour hydrates.
Prep the apple: Toss the finely diced apple with lemon juice.
Pat dry with a paper towel to reduce extra moisture. Fold apple pieces into the batter. Add nuts if using.
Fill the cups: Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Sprinkle with the cinnamon “sugar” topping if you like.
Bake: Bake for 18–22 minutes, or until the tops are set and a toothpick comes out with a few moist crumbs. Do not overbake.
Cool: Let muffins cool in the pan for 10 minutes, then move to a rack to cool completely. They firm up as they cool.
Serve: Enjoy warm with a pat of butter, or at room temperature.
They pair perfectly with coffee or tea.