Pre-heat the oven to 190°C / 170°C fan assisted / gas mark 5 / 375°F.
Line two large baking trays with baking parchment
Add the psyllium husk into a small bowl and whisk in the milk until combined. Place aside for 10 minutes as the mixture turns to a thick gel consistency.
Whisk the flours together in a large mixing bowl.
Peel the hard cold butter directly into the flour mixture.
Rub the flour and butter together to create rough breadcrumbs.
Add the sugar, baking powder and salt and mix well.
Stir in the sultanas.
Make a well in the centre of the dry ingredients and pour in the whisked eggs, followed by the apple cider vinegar and psyllium husk gel.
Mix the wet ingredients into the dry ingredients first with a silicone or wooden spoon to combine. Switch to using your hands when the spoon is no longer adequate and bring together until a loose dough starts to form.
Turn the loose dough out onto a clean work surface and finish bringing the dough together until it is no longer sticky. Smooth out into a large ball.
Press the dough out into an even round, 3cm (1- 1¼ inches) thick.
Press out circles of dough using a 7cm (2 ½ inches) fluted or round cutter. You will need to re-roll and re-pat the dough to use up all of the dough which should be enough for 12 scones.
After you have cut out six scones then arrange those
onto one of the lined baking trays.
Whisk the egg yolk with the caster sugar for the baking glaze then brush over each of these six scones.
Place the baking tray in the middle of the pre-heated oven and bake for 20 minutes.
Whilst those scones are baking you can cut out the scones from the rest of the dough. There should be a further six which you can arrange on the second baking tray.
Once the first batch of scones are out of the oven then you can glaze and bake the second batch.
Allow the scones to rest for five minutes on the baking tray before transferring to a cooling rack to cool completely.