Prepare the Blueberry Base:
Add blueberries, sugar, and orange zest to a saucepan.
Cook over medium heat until the blueberries break down and thicken (about 7 minutes).
Let the mixture cool, then divide evenly among ramekins.
Make the Custard:
In a bowl, whisk the egg yolks and sugar until pale and creamy.
In a saucepan, heat the heavy cream and vanilla over medium heat until it just starts to bubble at the edges.
Slowly pour the warm cream into the egg mixture, whisking constantly to prevent scrambling.
Optional: Strain the custard through a fine-mesh sieve for extra smoothness.
Pour the custard over the blueberry layer in the ramekins.
Bake Using a Water Bath:
Place ramekins in a deep baking dish. Pour hot water into the dish until it reaches halfway up the ramekins.
Bake at 160°C (320°F) for 35-40 minutes, until the custard is just set with a slight jiggle in the center.
Cool at room temperature, then refrigerate for at least 4 hours (or overnight).
Caramelize the Sugar Topping (Torch or Broiler Method):
Sprinkle 1 tsp caster sugar over each custard.
Torch Method: Use a kitchen torch to melt and caramelize the sugar until golden brown.
Oven Broiler Method: Place ramekins under the broiler for 3-5 minutes, rotating as needed for even caramelization.
Let the caramel cool and harden for 2 minutes before serving.