Place the all purpose flour, whole wheat pastry flour, brown sugar, baking powder and salt into a food processor, and blend to combine.
Add the egg yolk and all but 1 Tbsp of the egg white to the food processor along with the butter and milk, and pulse until a crumbly dough has formed.
Wrap the dough tightly in plastic wrap, and chill in the fridge for 30 minutes.
Roll the chilled dough out thinly onto a lightly floured surface.
Cut the dough into 2 ¾ inch circles with a cookie cutter.
Preheat the oven to 400 degrees.
Place a bit of filling on one side of each circle of dough.
Fold the dough over the filling, folding it to pinch it closed around the edges.
Layer the empanadas on a parchment lined baking sheet, and poke the top of each one with the tines of a fork in the center.
Brush the empanadas with the egg white, and bake for 20 minutes or until cooked through