01 - Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper, allowing some paper to overhang for easy bar removal.
02 - In a large mixing bowl, combine flour, sugar, baking powder, and salt. Cut in cold butter using a pastry cutter or fork until the texture is crumbly. Whisk milk and egg together in a separate bowl, then pour into dry ingredients, stirring just until the dough forms.
03 - Press two-thirds of the biscuit dough evenly into the bottom of the lined pan. Set aside the remaining dough for topping.
04 - Melt butter in a skillet over medium heat. Add diced apples, brown sugar, cinnamon, nutmeg, and lemon juice. Sauté until apples are just tender, approximately 5 to 6 minutes. Sprinkle cornstarch over apples and cook, stirring, an additional 1 to 2 minutes, until mixture thickens. Remove from heat.
05 - Spread cooked apple filling evenly over the biscuit base. Drizzle caramel sauce across the apples.
06 - Drop spoonfuls of the reserved dough over the caramel and apple layer, allowing some filling to remain visible.
07 - Transfer pan to the oven and bake for 30 to 35 minutes, until the top is golden brown and the filling is bubbling.
08 - Immediately after baking, brush the surface of the bars with melted butter. Mix sugar and cinnamon in a bowl, then sprinkle this mixture evenly across the top.
09 - Allow bars to cool completely in the pan before lifting them out with the parchment overhang. Slice into individual pieces and serve.