Heat milk and cream in a saucepan until simmering.
Whisk caster sugar and egg yolks until pale and fluffy.
Pour warm cream into the egg mixture while whisking continuously.
Cook the mixture over low heat until it coats a spoon.
Stir in chopped chocolate bars until melted and chill the custard.
Churn the chilled custard in an ice cream machine until frozen.
Rub flour, butter, and 25g caster sugar into breadcrumbs.
Mix in one egg yolk to form a dough and knead until smooth.
Flatten dough to a 2cm disc and chill for 30 minutes.
Mingle blueberries, vanilla, cornflour, lemon juice, and remaining sugar.
Roll pastry to 5mm thickness and cut eight 12cm discs.
Fold pastry edges on parchment, brush with egg yolk, and fill with berries.
Sprinkle with demerara sugar and chill for 30 minutes.
Bake at 200°C for 20-25 minutes until golden.
Serve warm galettes with a scoop of chocolate ice cream.