Preheat oven 180°C fan forced. Grease and line 3cm deep, 18x28cm base slice pan.
Combine the flour, custard powder and 1/2 cup of the sugar in a large bowl. Add the butter and use fingertips to rub it into flour mixture until mixture resembles fine breadcrumbs. Make a well in centre. Combine egg yolk and milk, add to flour mixture. Stir to a soft dough. Remove 1/3 (approx 1 cup/210g).
Press the remaining two-thirds of mixture evenly over the base of prepared pan. Scatter over the blueberries, sprinkle with remaining ¼ cup sugar. Crumble the reserved mixture over the blueberries. Bake for 40-45 minutes or until golden. Cool for 30 minutes. Remove from pan.
For the lemon drizzle, combine the icing and enough lemon juice to form a thick icing. Drizzle over the slice, allow to set before cutting into pieces.