To make the pre-ferment: In a bowl, combine the yeast, 1 teaspoon buckwheat flour, and 1 teaspoon sugar. Add the warm milk. Let the mixture sit and bubble for 30 minutes.

Preheat your oven to 120°F (50°C).

In the bowl of a stand mixer, combine the buckwheat flour, chestnut flour, cornstarch, tapioca starch, sugar, psyllium husk, salt, and lemon zest. Mix with a spatula until well combined.

Add the beaten egg, orange blossom water, softened butter, and the pre-ferment. Stir with a spatula.

Add the xanthan gum and mix well with the spatula.

Knead with the stand mixer (dough hook) for 1 minute on speed 1, then 7 minutes on speed 4, and 4 minutes on speed 6.

While the dough is kneading, prepare your loaf pan by greasing it and lining it with parchment paper.

Place the dough onto a well-oiled silicone baking mat. Oil your hands and shape the dough. The dough will be very sticky and soft—this is normal. Keep your hands well-oiled to handle it properly.

Shape the dough into a log and cut it into 3 equal pieces.

Place the pieces in the prepared pan. Cover the pan with plastic wrap, turn off the oven, and let the dough rise for 2 hours, until doubled in size.

After 2 hours, preheat your oven to 350°F (180°C). Meanwhile, brush the brioche with egg yolk and sprinkle with pearl sugar.

Bake for 40 minutes. After 20 minutes, cover the brioche with aluminum foil to prevent burning, then continue baking for another 20 minutes.

Turn off the oven and remove the brioche. Let it cool completely.

This brioche is very soft and fluffy. Store in an airtight container or bag.