For the crepes:
1) Whisk the flour, sugar, lemon zest and salt in a medium bowl. In a measuring cup, whisk together the milk, egg and 15g butter. Slowly whisk the egg mixture into the flour mixture until smooth.
2) In a heavy small saute pan or crepe pan, lightly brush the pan with a layer of melted butter and heat over medium-high heat. Working quickly, pour in about 45ml of batter and tilt the pan to coat. Cook for 1 or 2 mins and then flip with a thin metal spatula to finish cooking the other side, another min. Repeat with remaining batter. Keep the crepes warm while preparing the rest of the dessert.
3) Spread some of the blueberry filling in a line down the centre of each crepe and roll up. Spoon a few tbsp of the custard sauce on each dessert plate and lay 2 crepes on top. Sprinkle with icing sugar and serve.
For the blueberry filling:
1) Heat the jam in a small pan over low heat or microwave until warm and syrupy. Whisk in the lemon juice and set aside.
For the custard sauce:
1) In a small saucepan, whisk the sugar, cornflour and salt together until well blended. Whisk in the milk and cook over low heat until almost boiling, then remove from the heat. In a small bowl, whisk together the egg yolk and vanilla until well blended. Add a couple tbsp of the hot milk from the saucepan and whisk to temper the egg. Add resulting egg mixture to the rest of the warm milk in the pan, whisking to combine. Cook over low heat, stirring continuously, until the sauce begins to thicken (do not boil). Let the sauce cool slightly while assembling the crepes.