Preheat your oven to 350°F (175°C). Line an 8x4-inch loaf pan with parchment paper, leaving overhang on the sides for easy removal.
In a large bowl, whisk together the almond flour, psyllium husk powder, baking powder, and salt until well combined with no lumps.
Add the eggs, melted butter, and apple cider vinegar to the dry ingredients. Stir everything together until a thick batter forms.
Pour in the boiling water and mix quickly with a spatula for about 30 seconds. The dough will thicken rapidly and become slightly sticky, almost like a thick muffin batter. Work fast here since the psyllium husk absorbs water quickly.
Transfer the dough into the prepared loaf pan. Wet your hands lightly and smooth the top, shaping it into a gentle dome.
Bake for 55 to 65 minutes, until the top is deep golden brown and the loaf sounds hollow when tapped. If the top browns too quickly, tent loosely with foil after 40 minutes.
Remove from the oven and let the bread cool in the pan for 10 minutes. Then lift it out using the parchment overhang and transfer to a wire rack. Let it cool completely before slicing, at least 1 hour. This bread slices best when fully cooled.
