Step 1

Preheat the oven to 350°F (180°C). Grease the loaf pan with 2 teaspoons of the butter, then dust the pan with 2 teaspoons of the coconut flour, tapping out any excess.

Step 2

Place the remaining butter in a small saucepan and melt it over medium-low heat. Let simmer, stirring occasionally, until golden brown and nutty smelling, 5 to 8 minutes. Set the browned butter aside to cool.

Step 3

In a large bowl, whisk together the remaining coconut flour, the almond flour, ground flaxseed, baking powder, xanthan gum, cinnamon, nutmeg, and salt. Set aside.

Step 4

In another large bowl, combine the eggs and sweetener and beat with an electric mixer on medium speed, until fluffy and lighter in color, about 2 minutes. Reduce the mixer speed to low and add the banana and vanilla extracts, apple cider vinegar, and browned butter.

Step 5

Add the flour mix to egg mix in three parts, alternating with sour cream, beginning and ending with flour. Mix until a fully incorporated, thick batter, 2 to 3 minutes. Fold in half of the walnuts.

Step 6

Pour the batter into the prepared loaf pan. Dip a spoon in water, then use the back of the wet spoon to smooth the batter. Sprinkle the remaining walnuts on top, then bake until the top is deep golden and a toothpick inserted into the center of the loaf comes out clean, 50 to 55 minutes. After 30 minutes of baking, check the bread; if you see deep golden-brown areas on the crust and walnuts, gently tent the loaf pan with aluminum foil for the remainder of baking. Let cool in the pan on a wire rack for 15 minutes.

Step 7

Carefully remove the bread from the pan and let it cool completely on a wire rack before slicing, 50 to 60 minutes. Serve.