In a small saucepan, gently heat the milk until just warm to the touch. Add the yeast, stir briefly and allow it to sit for 10 minutes until lightly foamy.
Pour the activated milk into the bowl of a stand mixer. Add the eggs and whisk until well combined.
Add the plain flour, raw sugar, salt and xanthan gum. Using the paddle attachment, mix on low speed for 2 minutes until a thick, smooth batter forms.
Pause and scrape down the sides of the bowl. Increase the speed to medium and begin adding the softened butter, two tablespoons at a time, allowing each addition to fully incorporate before adding the next. Continue mixing for 2–3 minutes until fully combined. The mixture will look more like a thick cake batter than a traditional dough.
Scrape down the bowl to ensure the butter is evenly distributed. Cover the bowl tightly with cling wrap and place it in the fridge to rest for at least 1 hour.
Preheat the oven to 165°C and lightly oil spray a small loaf tin.
Remove the chilled dough from the fridge. With lightly floured hands, roll the dough into a loose log shape and divide into 8 portions using a dough scraper. Gently knead and shape each portion into a smooth ball, use extra flour if needed. Keep the dough covered with a damp tea towel as you work to prevent it from drying out.
Arrange the dough balls snugly into the prepared loaf tin. Brush the whisked egg over each brioche bun for a golden colour.
Bake on fan-forced for 30-35 minutes until golden and a skewer comes out clean. If the brioche starts to brown too quickly, loosely cover with foil.
Remove from the tin straight away and allow it to cool for 20 minutes before slicing.
