Preheat the oven to 350°F.

In a stand mixer, cream the softened butter and sugar until pale and fluffy (3-5 minutes on medium-high speed).

Add the flour, cornstarch, and salt. Pulse until just combined.

Pat the shortbread into a parchment-lined 9×13 inch pan until it forms an even layer.

Bake for 45-55 minutes until golden brown (check after 35 minutes if using a metal pan).

After baking, reduce the oven temperature to 300°F for baking the bars.

While the shortbread bakes, brown the butter in a large skillet over medium heat until golden and toasty.

Stir in the sugars, salt, cinnamon, and vanilla.

Add the apple slices and lemon zest. Cook over medium heat for 8-10 minutes until slightly softened. Set aside to cool.

In a bowl, combine flour, cinnamon, and brown sugar.

Pour in the melted butter and mix with a fork until large crumbs form. Set aside.

Spread the apple pie filling evenly over the cooked shortbread.

Sprinkle the crumble evenly over the apple layer.

Bake at 300°F for 30 minutes.

Increase the oven temperature to 350°F and bake for an additional 15 minutes until golden brown.

In a medium bowl, mix powdered sugar and vanilla. Gradually whisk in milk.

Once the bars are cooled, drizzle the glaze over the top using a spoon or piping bag (a Ziploc bag with the corner snipped off works too).