Position oven rack in the middle and preheat to 350°F (175°C), then line a 9x9 inch (23x23cm) light-colored aluminum baking pan with two sheets of parchment paper running in opposite directions, each with at least 2 inches (5cm) of overhang on all sides.

In a large mixing bowl, whisk together 2 cups (240g) all-purpose flour, ¾ cup (150g) granulated sugar, and ¼ tsp (1.5g) fine sea salt, then add 1 tsp (5ml) pure vanilla extract and toss briefly with a fork.

Add ¾ cup (170g) cold unsalted butter cubes all at once and work into the flour mixture using a pastry blender or fingertips, pressing and smearing until the mixture looks shaggy and uneven with butter pieces ranging from pea to almond size — do not overwork.

Transfer approximately two-thirds of the mixture (roughly 310g) to the prepared pan and press firmly and evenly across the entire base using the flat bottom of a measuring cup, paying attention to the corners, until the base is approximately ¼ inch (6mm) thick with no gaps or thin spots.

In a medium bowl, combine 3 cups (480g) peeled and sliced fresh yellow peaches, 3 tbsp (36g) granulated sugar, 1 tbsp (8g) cornstarch, 1 tsp (5ml) fresh lemon juice, and ¼ tsp ground cinnamon, then toss gently with a silicone spatula for approximately 20 slow folds until every peach piece is coated.

Scatter the coated peach mixture evenly over the pressed shortbread base, then drop 3 tbsp (60g) seedless raspberry jam in small teaspoon-sized dollops across the peach layer — approximately 9 to 12 scattered dots — do not spread the jam.

Add ½ cup (45g) old-fashioned rolled oats to the reserved one-third of the shortbread mixture and toss with a fork until distributed, then scatter the oat crumble loosely over the peach filling without pressing it flat — leave it in natural clumped state.

Bake at 350°F (175°C) for 40 to 45 minutes, starting to check at 38 minutes, until the crumble peaks are deep golden brown and the fruit filling is visibly bubbling through the gaps in the crumble at the center of the pan — not just at the edges.

Remove from the oven and cool in the pan on a wire rack for exactly 60 minutes at room temperature, then transfer the pan to the refrigerator and chill for a minimum of 2 hours before slicing.

After chilling, grip the parchment overhang on two opposite sides and lift the slab straight out of the pan onto a flat cutting surface, then use a sharp chef's knife wiped clean between every cut to slice into 3 columns and 4 rows for 12 bars approximately 2¼ x 2¼ inches (5.7 x 5.7cm) each.

If glazing: sift 1 cup (120g) powdered sugar into a small bowl, add 2 tbsp (30ml) heavy cream and whisk until smooth, adjust consistency with additional cream or powdered sugar, then drizzle in a zig-zag pattern over the sliced bars from a height of about 8 inches (20cm) and allow to set for 10 minutes before serving.