Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line the bottom with parchment paper, leaving an overhang on the long sides for easy removal.
In a large bowl, whisk together the almond flour, coconut flour, erythritol, baking powder, baking soda, cinnamon, and salt.
In a separate bowl, whisk the eggs, melted butter, sour cream, banana extract, and vanilla extract until smooth. Pour the wet ingredients into the dry ingredients and stir until a thick batter forms with no dry pockets remaining.
Fold in the chopped walnuts, reserving a small handful to sprinkle on top if desired.
Pour the batter into the prepared loaf pan and smooth the top. Scatter any remaining walnuts over the surface.
Bake for 45 to 55 minutes, until the top is deep golden brown and a toothpick inserted in the center comes out clean. If the top browns too quickly, tent loosely with foil after 30 minutes.
Let the bread cool in the pan for 15 minutes, then lift it out using the parchment overhang and cool completely on a wire rack before slicing.
