Make the white chocolate custard: Transfer the corn starch and the sugar to a small pot and stir to mix. Add the eggs and whisk until everything is combined. Add the milk and the salt and cook over medium heat, whisking constantly, until the mixture starts to bubble. Let it bubble for 1 minute and take it off the heat.
Add the white chocolate and the vanilla and wait for 1 minute for the chocolate to start melting. Stir until the chocolate is completely melted and the cream is smooth. If you notice you have any lumps, you can pass the cream through a sieve, or blend it to make it smooth. Transfer to a bowl, cover with a wrap in a way that the wrap touches the surface and refrigerate.
Make the chestnut cream: Transfer the chestnuts and the sweet chestnut spread to a food processor and process until smooth. You may have to stop and scrape the side a few times. Add the water gradually until you get a smooth, spreadable cream.
Built the Mont Blanc: Pour the milk in a shallow bowl and briefly dip half of the savoiardi biscuits. Divide the savoiardi between the glasses and spread a teaspoon of chestnut cream to each glass. Then, divide half of the custard between the glasses.
Repeat with another layer of savoiardi dipped in milk and cover with the rest of the custard. Using a spoon or an offset spatula give the custard a pyramid shape.
Transfer the chestnut cream to a piping bag with a star tip or a thin round tip at the end and pipe the chestnut cream over the custard.
Store in the fridge for a few hours to chill. Sprinkle with icing sugar and (optionally) decorate with white chocolate curls before serving.
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