First, make the custard: Heat the milk in a saucepan. Whisk the egg yolk with the vanilla sugar.

Pour the hot milk over the egg yolk and sugar mixture, then return everything to the saucepan.

Cook over medium heat for about 10 minutes, stirring regularly, until it thickens. Let cool to room temperature, then refrigerate.

Make the floating island: Beat the egg white until stiff peaks form, then shape into quenelles. Microwave for 20 seconds.

Assemble the dessert: Place the island on top of the custard once it is cold.

Finish with the caramel: In a saucepan, add the butter, water, and sugar. Heat over medium heat.

Once the butter has melted, wait for the mixture to boil, stirring regularly. When it turns light brown, remove from heat and add the milk.

Return to the heat, stirring constantly. Wait until the texture becomes creamy, then let cool in a bowl.