Start with the choux pastry: In a saucepan over medium heat, combine the water, sugar, and butter. Bring to a boil.

Remove from heat and add the flour all at once.

Return to the heat and stir until the dough forms a ball and dries out slightly.

Transfer the dough to a mixing bowl and let it cool for a few minutes.

Add the beaten egg in three additions, mixing well after each.

Preheat the oven to 400°F (200°C). Transfer the dough to a piping bag and pipe into rings. Sprinkle with sliced almonds. Bake for about 25 minutes.

For the mousseline cream: Take the butter out of the fridge 2 hours ahead of time.

In a saucepan, heat the milk and split vanilla bean over medium heat.

In a bowl, whisk together the egg yolk and sugar. Add the sifted flour and cornstarch, then mix.

Pour the hot milk over the mixture and stir. Return to the saucepan and cook, stirring, until thickened to make a pastry cream.

Once ready, add 1 tablespoon (13 grams) of butter and mix. Let cool until the remaining butter and the cream are at the same temperature.

When both are at the same temperature, beat the cream and gradually add the remaining butter.

Continue beating for about 7 to 10 minutes until the cream is smooth and creamy.

Fill the choux pastry with the cream and refrigerate.