• In a large bowl, stir together the flours, sugar, and salt to combine.
• Add the butter slices into the flour, and rub the butter into the flour with your fingertips until the mixture resembles sand with large walnut-sized butter chunks. 
• Stir together the water and apple cider vinegar. Make a well in the center of the flour mixture and add the water mixture. Add the sugar and dissolve by stirring with finger tips. Finally, add the beaten egg yolk.
• Gradually incorporate the flour mix into the liquid, starting from the inside of the well. When the liquid has disappeared, use both hands to mix the pastry until it has an even consistency. Do not overwork it – when all the ingredients are combined, and just holding together, it is ready.
• Unless processing with additional step, wrap in cling wrap, and chill for 30 minutes or up to 2 days

additional step for an extra flakey crust

• Dump the dough out onto a counter, divide it roughly into palm sized portions, and "fraisage" by dragging a portion of dough across the surface using the heel of your hand. Scrape up the dough (a metal bench scraper works well here), gently press it into a ball and flatten into a disc.
• Wrap in cling wrap, and chill for 30 minutes or up to 2 days

when ready to use

• Let the pie crust come to room temperature for 10-15 minutes before rolling out and using as per recipe instructs.