Preheat the oven to 350F and grease a muffin tin or large loaf pan with nonstick spray.
In a large bowl, mash the banana with a fork then add in the melted butter, eggs, egg yolks, and vanilla. Stir until well combined.
In a separate bowl, combine the almond flour, flax meal, erythritol (or other sweetener), salt, and baking powder. Stir until it's all mixed together.
Gradually combine the dry ingredients with the wet, stirring after each addition, until they are well incorporated.
Transfer batter to muffin tin or loaf pan, and bake 20 minutes for muffins and 30-35 minutes for a loaf pan.
