Preheat your oven to 350 degrees F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides so you can lift the bars out cleanly later. Lightly butter the parchment.

Toast the oats for the crumble topping. Pour the oats into a dry skillet over medium heat and stir them around for 4 to 5 minutes until they smell nutty and turn a shade darker. Pour them onto a plate to cool. This step adds real depth to the topping and is worth the few extra minutes.

Make the shortbread base. In a large bowl, whisk together 1 cup of the flour, the powdered sugar, and the salt. Add the cold cubed butter and use your fingertips to press it into the flour until the mixture looks like coarse, damp sand with a few pea-size pieces of butter remaining. Pour this into the prepared pan and press it into an even, firm layer using the bottom of a measuring cup. Bake for 15 minutes, until the edges just start to turn pale gold. Pull it out and set it aside.

While the base bakes, make the oat crumble. In a small bowl, stir together the toasted oats, the remaining 1/4 cup of flour, the brown sugar, chai spice, and cinnamon. Drizzle the melted butter over the top and toss with a fork until the mixture clumps together in small clusters.

Spread the apple butter evenly over the warm shortbread base, going almost to the edges. Use the back of a spoon or an offset spatula. Scatter the oat crumble evenly over the apple butter, covering it in a loose, uneven layer.

Bake for 35 minutes, until the crumble topping is golden brown and set. The apple butter layer may bubble around the edges, which is fine. Remove from the oven and let the bars cool completely in the pan, at least 1 hour. Do not rush this step. Warm bars will crumble when you cut them.

Use the parchment overhang to lift the slab out of the pan onto a cutting board. Cut into 16 squares with a sharp knife. Wipe the knife between cuts for cleaner edges.