• Preheat the oven to 300℉ and line bottom and sides of a 13 x 9" baking pan with a parchment paper sling so you can easily pull the bars out after baked.

Shortbread Base

• Place the softened butter into a stand mixer (with paddle attachment) and turn on high speed for 4 minutes until it turns pale and slightly whipped. Scrape down the sides.
  1 cup unsalted butter
• Add in the sugar, vanilla, and salt and mix on medium speed for one minute. Scrape down the sides again.
  ½ cup granulated sugar, 2 teaspoon vanilla paste or extract, ¼ teaspoon kosher salt
• Add in the flour and mix on low until combined. The dough will look crumbly at first, then will come together and look slightly sticky (this is normal).
  2 cups all-purpose flour
• Scatter pieces of the dough all around the pan and press it (with fingertips) into the edges and middle until all the dough comes together to cover the pan.
• Bake for 15 minutes, then remove from oven and let cool on a wire rack. Turn the oven up to 350℉.

Apple Layer

• While the crust is baking, peel, core, and slice apples (⅛ inch thick). In a large bowl, combine the apples, flour, sugar, cinnamon and nutmeg and lightly toss until the apples are fully coated.
  3-4 medium apples, ¼ cup all-purpose flour, ¼ cup granulated sugar, 2 teaspoons ground cinnamon, ½ teaspoon ground nutmeg

Crumble Topping

• Mix the flour, brown sugar, oats, salt, and melted butter with a fork until just combined and crumbly.
  1 cup all-purpose flour, 1 cup rolled oats, 1 ½ cup light brown sugar, ¼ teaspoon kosher salt, ½ cup unsalted butter
• Sprinkle one third of the topping over the shortbread, then layer the apples over the crumble in slightly overlapping horizontal lines. Gently press down on the apples with your hands.
• Once the apples are arranged, sprinkle the remainder of the topping evenly on top and bake for 30 minutes.
• Let cool completely on a wire rack, then refrigerate for 2 hours. Using the parchment slings, remove the full slab of bars to a cutting board and cut into 12 or more pieces and serve.