Add lemon juice to the milk to make buttermilk and set aside.
Cut refrigerated butter into small cubes.
Combine flour and salt in a medium bowl. Cut in the cold butter using a pastry cutter or fork , working the dough as little as possible until course, pebble-size pieces form. Add the buttermilk mixture and mix the dough together until it combines just enough to stick together when you pinch it.
Divide the dough into two balls and form each half into a thick disk (so it's round with a flat top - ready to be rolled out later). Wrap tightly in plastic wrap. Set the dough in the fridge for 20-30 minutes.
Preheat oven to 400* F.
In a small bowl, combine pumpkin, well-mashed banana, brown sugar, and pumpkin pie spice.
Once chilled, roll the pastry dough out with a rolling pin on a well-floured surface until it is a little less than 1/4 inch thick. Cut circles 4 to 5 inches wide, using a round pastry cutter or tracing a round object with a knife. Combine all scraps in a ball and roll out once more to repeat.
Evenly distribute the pumpkin mixture into the middle of each of the pastry circles. Fold the circles over and crimp around the edges to seal pastry, using a fork or your fingers. Place at least an inch apart on a baking sheet.
Brush each pastry with egg wash, then sprinkle with turbinado sugar.
Bake for 20 minutes, or until nicely browned. Remove from the sheet after a few minutes and place on a cooling rack for at least 20 minutes before serving.