In a large mixing bowl, combine the flour, sugar, baking powder, and salt.
Using your fingers or pastry cutter, cut in the butter until the mixture is crumbly.
Add the egg yolks and milk, mix with spatula until the dough is just combined.
Divide pie dough into 6 pieces; roll each into a ball. Cover and chill until while filling is prepared.
To make filling, peel and core apples; slice thin, then cut slices in half. Heat butter in a skillet over medium heat until golden brown. Whisk salt, brown sugar, cinnamon . Add apples and stir to coat. Cook at a low simmer, stirring occasionally, until apples have softened. Set aside and allow to cool.
When apple filling has cooled, arrange oven racks in order to bake in the bottom level. Preheat oven to 400-degrees F and line a large baking sheet with baking parchment.
On a floured surface, roll each dough ball into a circle about 5-inches in diameter. Place equal amounts of apple filling in center of each dough circle (about 1/4 cup per circle). Fold dough over filling. Use a fork to crimp edges together to seal hand pie. Place hand pies onto parchment lined baking sheet.
In a small bowl, whisk together the egg and milk. Brush tops of each hand pie with egg mixture; sprinkle tops evenly with granulated sugar. Cut small vents on the top.
Bake at 400-degrees F for 15 minutes, then lower oven temperature to 350-degrees F and bake for another 10-15 minutes, until golden brown and bubbly. Let cool slightly before serving.