In a small bowl, combine the warm water and yeast. Let it sit for 5-10 minutes until foamy.
In a large bowl, whisk together almond flour, coconut flour, psyllium husk powder, baking powder, and salt.
In a separate bowl, whisk the eggs, melted butter, and erythritol until well combined.
Add the foamy yeast mixture to the wet ingredients and stir.
Pour the wet ingredients into the dry ingredients and mix until a dough forms. It will be sticky.
Cover the bowl with plastic wrap and let the dough rise in a warm place for about 30-45 minutes, or until doubled in size.
Preheat your oven to 350°F (175°C). Grease and flour a loaf pan (8.5 x 4.5 inches) or line with parchment paper.
Punch down the dough and shape it into a loaf. Place it in the prepared loaf pan.
In a small bowl, whisk together the egg yolk and heavy cream for the egg wash.
Brush the top of the loaf with the egg wash.
Bake for 30-40 minutes, or until golden brown and cooked through. The internal temperature should reach 200-210°F (93-99°C).
Let the bread cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
Notes
This keto brioche bread is best served fresh. Store any leftovers in an airtight container at room temperature for up to 2 days, or freeze for longer storage.