Preheat the oven to 375°F. Line two 9 by 5-inch loaf pans with parchment paper, with the edges left hanging over the sides. Spray the parchment with cooking spray.
Place the eggs, butter, water, extracts, and stevia in the bowl of a stand mixer (or a large bowl if using an electric hand mixer) and mix until well combined.
In a large mixing bowl, whisk together the brown sugar sweetener, coconut flour, ground flaxseed, baking powder, cinnamon, xanthan gum, and salt. Pour the dry ingredient mixture into the wet ingredient mixture and blend until combined.
Divide the batter between two bowls and stir half of the chocolate chips or nuts, if using, into each bowl. Transfer the batter to the prepared loaf pans.
Bake for 40 to 45 minutes, or until a toothpick inserted in the center of a loaf comes out clean. Remove from the oven and allow to cool completely in the pans.
Once cool, remove from the pans by lifting up on the parchment paper. Slice and serve one loaf and freeze one loaf.
Storage: Wrap tightly in aluminum foil and refrigerate for up to 1 week or place in a zip-top freezer bag and freeze for up to 1 month.