First make the filling by melting the butter in a large saucepan over a low heat.
Once fully melted, pour in the rest of the filling ingredients, stirring thoroughly to make sure everything is coated in butter. Set aside for 1 hour and make the pastry in the meantime.
To make the pastry, combine the flours, chia seeds and salt into a large mixing bowl.
Grate the cold butter into the flour then gently mix together with your fingertips so the mixture turns quite shaggy.
Stir in the beaten egg with a fork.
Add the cold milk one tablespoon at a time and start to bring the dough together with a pastry scraper. It should start to form quite quickly. It should be slightly sticky to the touch.
Tip the dough onto the work surface and quickly bring the ball into a round ball with your hands. You don’t really need to work the pastry as there’s no gluten to activate.
Wrap the pastry ball in cling film and flatten it slightly to make it easier to roll out when ready.
Place in the fridge for an hour to chill.
When you are ready to assemble the Eccles Cakes pre-heat the oven to 170°C.
Take the pastry out from the fridge, remove the cling film then tear the pastry in half to make it easier to roll out. Use the extra oat flour to dust the worktop and the rolling pin.
Roll out the pastry to about 3mm thickness then cut out circles of pastry using a 12cm round pastry cutter.
Spoon in two teaspoons of filling into the centre of each round.
Whisk the egg yolks and whole milk together into a small cup.
Damp the edges of the pastry lightly with the egg yolk and milk mixture, then bring all the sides together, pressing firmly to seal.
Flip each Eccles cake over then gently roll out with the rolling pin to flatten slightly so the filling is just showing below the surface.
Set the Eccles Cakes onto a large baking tray, using a sharp knife make two little slits in the centre of each cake then brush each surface with the egg yolk and milk mixture and sprinkle with demerara sugar.
Bake for 15 minutes until golden.