Prep the pan: Preheat the oven to 350°F (175°C). Line a 8.5 x 4.5-inch loaf pan with parchment and lightly grease the sides.
Mix dry ingredients: In a bowl, whisk 2 cups fine almond flour, 2 teaspoons baking powder, and 1/2 teaspoon salt.
If using 1 teaspoon psyllium husk powder or 1/4 teaspoon xanthan gum, whisk it in now.
Whisk wet ingredients: In another bowl, beat 4 large eggs until frothy, about 30–45 seconds. Whisk in 1/4 cup melted butter (or avocado oil) and 1 teaspoon apple cider vinegar.
Combine: Stir the wet into the dry until just combined. If adding seeds or seasonings (up to 1/4 cup total), fold them in.
The batter will be thick but spreadable.
Pan and smooth: Transfer to the loaf pan. Smooth the top with a spatula and gently tap the pan to release air pockets.
Bake: Bake for 30–38 minutes, until golden and a toothpick comes out clean or with a few moist crumbs. The top should spring back when gently pressed.
Cool: Let the loaf cool in the pan for 10 minutes, then lift it out and cool fully on a rack. Do not slice while warm—it needs to set for clean slices.
Slice and serve: Use a serrated knife for neat slices.
Toast for extra crunch and flavor.