Use a fork to bring to dough together. Try to moisten all of the flour bits. On a lightly floured work surface, dump out the dough mixture. It will be moist and shaggy. Sprinkle generously with flour. Shape dough into a disk and wrap in plastic wrap. Allow dough to rest in the fridge while you assemble the filling. Dough is easiest to roll out when it’s cold and rested.
Quickly break the butter down into the flour mixture, some butter pieces will be the size of oat flakes, some will be the size of peas. In a small bowl, stir together the egg yolk and buttermilk. Create a well in the butter and flour mixture and pour in the liquid mixture.
In a medium bowl, whisk together flour, sugar, and salt. Add cold, cubed butter and, using your fingers, work the butter into the flour mixture.