Step 1

Preheat the oven to 375°F. Grease a 9×13” baking dish with butter. Line the dish with a piece of parchment paper, pressing it flush to the bottom and up the sides, allowing about 2 inches of paper to hang over the edge.

Step 2

For the filling: heat a 12” cast-iron skillet over medium-high heat. Add the butter and let melt. Add the sliced apples. Saute for about 8 minutes until caramelized, stirring often. Add the brown sugar, cinnamon, and salt. Stir well and allow the sugar to melt. Simmer for about a minute. Turn off the heat and let cool.

Step 3

For the shortbread crust: add the butter, brown sugar, cinnamon, and salt to the bowl of a stand mixer fitted with a paddle attachment. Mix on medium speed until smooth and fluffy, about 3 minutes. Add the flour in two batches, mixing to incorporate. Scrape down the sides of the bowl as needed to create an even mixture.

Step 4

For the streusel topping: combine the flour, brown sugar, cinnamon, and salt in a mixing bowl. Toss to combine. Add the cubes of butter. Use your hands (or a pastry cutter) to cut in the butter, rubbing the butter between your fingers until all of the flour is absorbed and the streusel resembles coarse crumbs.

Step 5

To assemble: press the shortbread dough evenly into the bottom of the prepared baking dish to create one thin, even layer. Spread the apple filling (and all of the sauce from the skillet) over top. Crumble the streusel across the apples.

Step 6

Bake for 30-35 minutes, or until golden brown and set. Allow to cool at room temperature for about 45 minutes. Carefully use the parchment paper to lift the bars out of the baking dish. Use a serrated knife to cut the bars into 12 squares. Serve warm or at room temperature.