Gather all of your ingredients, weighing out the flour, both sugars and butter. For the apples, choose a variety of different textures, sweetness and tartness. Be sure to spray a half sheet pan with cooking spray, and line with parchment paper.Preheat the oven to 350℉. Peel and chop the apples, sprinkle with a little lemon juice to keep them from browning as you go. Add the apples and the granulated sugar to a pot over medium high heat and begin to cook down. Apples should reduce by about half, about 20-25 minutes, stirring every so often.
While the apples are cooking, prepare the shortbread crust. Stir the flour, brown sugar and cooled melted butter until the mixture comes together, then use your hands to ensure that it's slightly crumbly, sticking together when squeezed, but certainly not damp.
Press about 4-½ cups of the crust mixture into the bottom of the prepared sheet pan, using your fingers to secure the edges and a measuring cup to flatten the surface. Reserve the remainder to sprinkle on top before baking.Pop the crust into the oven for 10 minutes, then remove and cool. In the meantime, remove the cooked apples from heat and allow to cool. Consistency should be similar to very chunky applesauce.
In the bowl of a food processor, pulse the dates until you've created a thick paste.Depending on the size of your processor, you may wish to remove half of the pulsed dates, so you can mix them in with the apples in multiple batches. Add the cooled apples and the vanilla extract and process until you have a less chunky applesauce mixture, with some pieces of apple remaining. The mixture should not be smooth.
Spread the apple date mixture evenly over across the crust.Sprinkle the top generously with the remaining crust until it nearly covers the surface. Return the pan to the oven and bake for 20 minutes, or until the crumbled topping is lightly golden brown and the filling is bubbling.
Allow the bars to cool COMPLETELY before attempting to cut them. Ideally, after they've cooled enough that the pan is no longer warm, pop the bars into the refrigerator or freezer to firmly set. This will make removing the bars from the pan and cutting them significantly easier.A little trick is to lay a second sheet pan on top (the bottom of the pan pressed against the top of the bar layer), and flip the pans over, so when you remove the pan the bars were baked in, the parchment paper will be on top. Remove the parchment, gently place a clean cutting board on top, and flip, so the bars are now on the board and ready for cutting.Slice bars into rectangles, squares, or halve again into triangles. Serve chilled with a light dusting of powdered sugar. Store leftover bars in the refrigerator in an airtight container.