Preheat your oven to 350°F (180°C). Spray the bottom and sides of a 9x9 inch (22 cm) square cake tin and line it with baking paper, leaving some overhang for easy removal later.

In a food processor, combine the flour, lemon zest, caster sugar, and salt. Pulse 3-4 times to mix. Add the chilled, cubed butter and pulse until the mixture resembles pea-sized crumbs. If you prefer a hands-on approach, you can mix the crumble ingredients in a bowl using your fingers or a pastry cutter.

Add the egg yolk and cold water to the mixture. Pulse until the dough starts to come together.

In a large mixing bowl, combine the frozen blueberries, lemon juice, 2 tablespoons of caster sugar, and corn starch. Gently mix until the blueberries are well coated.

Press half of the crumble mixture firmly and evenly into the bottom of the prepared cake tin. Spread the blueberry mixture evenly on top.

Sprinkle the remaining crumble mixture over the blueberry layer.

Bake in the preheated oven for 1 hour, or until the top is golden brown. Keep an eye on it towards the end to ensure it doesn’t over-brown.

Once baked, allow the crumble to cool for 20 minutes in the tin. Dust with powdered sugar before slicing into 9 squares.