1-First Step: Prep the pan and apples. Start by lining a 9×13-inch pan with parchment paper. Leave a little overhang on the sides so you can lift the bars out later without a struggle. This small step makes slicing much easier and helps keep the bars looking neat. Next, make the apple filling. Place the thinly sliced apples in a bowl with the granulated sugar, cinnamon, lemon juice, and vanilla extract. Stir until the apple slices are coated. Set the bowl aside while you make the dough so the fruit has time to release a little juice.

2-Second Step: Preheat the oven. Preheat your oven to 325 degrees Fahrenheit. A lower baking temperature helps the shortbread bake evenly and keeps the top from browning too quickly before the center is ready. If your oven tends to run hot, you may want to check the bars a few minutes early. Ovens can be a little different, and that is perfectly normal.

3-Third Step: Make the shortbread dough. In a large bowl, beat the unsalted butter, light brown sugar, and granulated white sugar until smooth. The mixture should look creamy and well mixed. Then add the vanilla extract and salt, and stir again. Mix in the all-purpose flour until the dough starts to come together. It will be thick, which is exactly what you want. Do not overwork it, since that can make the shortbread tougher than it should be. A soft, even dough gives the best texture for apple shortbread bars.

4-Fourth Step: Build the crust. Divide the dough in half. Press one half firmly and evenly into the prepared pan to make the crust. Try to keep the layer smooth so it bakes evenly from corner to corner. Put the other half of the dough in the refrigerator while the crust bakes. This chilled dough will become the crumble topping later. Keeping it cold helps it hold its shape a little better in the oven.

5-Fifth Step: Bake the crust. Bake the crust for about 20 minutes, or until it is lightly browned around the edges and looks set in the center. It does not need to be fully deep golden at this stage. You just want it firm enough to support the apple layer. Once baked, let the crust cool slightly. This short cooling time helps the fruit layer sit on top more neatly instead of sinking into the base.

6-Sixth Step: Add the apple filling. Use a slotted spoon to spoon the apple mixture over the crust. The slotted spoon is helpful because it lets some of the extra liquid drain off, which keeps the bars from turning soggy. Spread the apples in an even layer so every bite gets a good amount of fruit. If you want a more rustic look, leave the apple layer a little uneven. That works just fine and still bakes beautifully.

7-Seventh Step: Add the crumble topping and finish baking. Crumble the reserved dough over the apple layer. Break it into small pieces and scatter it evenly across the top. Some larger and smaller pieces are both good, since that creates a nice mix of soft and crisp textures. Bake the bars for another 30 to 35 minutes, or until the top is golden brown. This is the sign that the bars are ready. The apple filling should be bubbling a little at the edges, and the top should look nicely baked without being pale. These bars are done when the top turns golden brown. That color tells you the shortbread is baked through and ready to cool.

8-Eighth Step: Cool before slicing. Set the pan on a wire rack and let the bars cool for 15 minutes. Then use the parchment paper to lift the whole slab out of the pan. Let it cool fully before slicing, since warm bars can crumble and fall apart. Once cooled, cut into squares or rectangles. Serve them plain, or add a light dusting of powdered sugar if you want a prettier finish for guests.