Prep the pan and oven: Preheat your oven to 350°F (175°C).
Line a 9x5-inch loaf pan with parchment paper, letting the edges overhang for easy lifting. Lightly grease any exposed sides.
Mix dry ingredients: In a large bowl, whisk almond flour, coconut flour, ground flaxseed, baking powder, baking soda, salt, cinnamon, and your sweetener until well combined and lump-free.
Combine wet ingredients: In a separate bowl, whisk the eggs, melted butter, almond milk, and vanilla. Add banana extract, or mash in the small portion of ripe banana if using.
Bring it together: Pour the wet mixture into the dry.
Stir with a spatula until just combined and smooth. If using nuts or chocolate chips, fold them in gently.
Fill the pan: Transfer the batter to the lined loaf pan. Smooth the top with a spatula.
Sprinkle a few extra nuts or chips on top if you like.
Bake: Bake for 45–55 minutes, or until the top is golden and a toothpick inserted in the center comes out mostly clean with a few moist crumbs. If the top browns too fast, tent loosely with foil for the last 10 minutes.
Cool completely: Let the loaf cool in the pan for 15 minutes, then lift it out using the parchment and cool fully on a rack. This step sets the crumb so it slices cleanly.
Slice and serve: Use a serrated knife to cut into thick slices.
Enjoy plain, with butter, or a smear of sugar-free nut butter.