Heat the oven to 350°F (175°C) with a rack in the middle. Lightly grease an 8-inch square metal pan and line it with parchment, leaving a 2-inch overhang on two sides.

In a large bowl, rub the lemon zest into the sugar with your fingertips until fragrant. Whisk in the flour, baking powder and salt.

Cut in the cold butter until the largest pieces are pea-sized. Whisk the egg yolk with the vanilla, drizzle over the mixture and toss with a fork until it holds together in clumps when squeezed.

Set aside about 1⅓ cups (250 g) of the crumble for the topping and refrigerate it.

Press the remaining crumble firmly and evenly into the pan (about ½ inch thick). Prick with a fork and bake for 15 minutes, until the edges are pale gold.

Meanwhile, whisk the filling sugar, cornstarch and salt together, then gently fold in the blueberries, lemon juice and zest.

Spread the filling over the hot crust to the edges. Squeeze the cold topping into big and small clumps and scatter over the fruit, leaving a few small gaps.

Bake for 45–55 minutes, until the topping is deep golden and the filling bubbles thickly in the center. Tent loosely with foil after 35 minutes if browning too fast.

Cool on a wire rack for 2 hours, then refrigerate for 1–2 hours. Lift out with the parchment and cut into 12 bars (or 9 large) with a sharp knife, wiping the blade between cuts.