Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
In a mixing bowl, combine the all-purpose flour, baking powder, salt, and granulated sugar. Mix well.
Add the cold, cubed butter into the flour mixture. Using a pastry cutter or your fingers, work the butter into the flour until it resembles coarse crumbs.
Pour in the whole milk gradually and stir until a soft dough forms. Be careful not to overmix.
In a separate bowl, combine the diced apples, ground cinnamon, and lemon juice. Stir until the apples are well coated.
On a floured surface, roll out the dough to about ½ inch thick. Cut the dough into squares or circles using a knife or biscuit cutter.
Place a spoonful of the apple mixture in the center of each dough piece, then fold the dough over to create a pocket. Seal the edges by crimping with a fork.
Arrange the biscuits on the prepared baking sheet. For added sweetness, mix the brown sugar and milk to create a glaze and brush it over the top of each biscuit.
Bake in the preheated oven for 12-15 minutes, or until golden brown and rising.
Remove from the oven and allow to cool slightly before serving.