Heat the oven to 350°F, then line a 9x13-inch pan with parchment and leave an overhang on the long sides for lifting. The parchment should sit flat with clean edges so the bars release easily.

Beat the softened unsalted butter and powdered sugar for 2 minutes, until light and creamy. You should see the mixture lighten visibly and look fluffy.

Mix in the vanilla extract, all-purpose flour, and fine salt just until a crumbly dough forms. Stop as soon as the dough comes together so it stays tender after baking.

Press the dough firmly and evenly into the pan, smoothing the surface. Bake for 18–20 minutes at 350°F until the edges are pale golden and the center looks dry.

Leave the oven on while you prepare the apple layer. The base should be hot when you add the apples.

Toss the peeled and thinly sliced Granny Smith apples with packed light brown sugar, all-purpose flour, ground cinnamon, ground nutmeg, and lemon juice until every slice is coated. Let the mixture sit so the sugar starts to dissolve and lightly clings to the apples.

Rub the old-fashioned rolled oats, all-purpose flour, packed light brown sugar, ground cinnamon, cold unsalted butter, and fine salt together with your fingers until it forms coarse clumps the size of peas and small marbles. The texture should look shaggy and chunky rather than powdery.

Spread the apple slices over the hot shortbread base in a tight, even layer, overlapping slightly so there are no gaps. The fruit should cover the entire surface.

Scatter the oat crumble over the apples and squeeze some into larger clusters as you go for craggy texture. Aim for an even blanket with visible clumps.

Bake at 350°F for 38–45 minutes, until the topping is deep golden and apple juices bubble thickly at the edges. The bars are ready when the center looks set and the crumble holds shape.

Cool in the pan for at least 1 hour so the fruit layer sets and the shortbread firms up. The surface should feel firm when touched lightly.

Lift out using the parchment overhang, then drizzle with warm caramel sauce and sprinkle with flaky sea salt. Cut into 24 bars once the bars are fully cooled for clean edges.