Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease with butter or coconut oil.
In a large bowl, whisk the almond flour, coconut flour, baking powder, baking soda, salt, and sweetener until no lumps remain. Even mixing prevents dense spots.
In a separate bowl, whisk the eggs, almond milk, melted butter (or oil), vanilla bean paste, vanilla extract, and apple cider vinegar until smooth.
Pour the wet ingredients into the dry ingredients. Stir gently with a spatula until just combined.
The batter should be thick but scoopable. Avoid overmixing to keep the crumb tender.
If using mix-ins like sugar-free white chocolate chips or chopped almonds, fold them in now.
Divide the batter evenly among the muffin cups, filling each about 3/4 full. Smooth the tops with the back of a spoon if needed.
Bake for 18–22 minutes, or until the tops are set and lightly golden, and a toothpick comes out with a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. Letting them cool helps them set and improves texture.