FOR THE SOAKING SYRUP
2.5 oz (75 g) xylitol sweetener
2/3 cup (150 ml) mineral water
1 tsp (5 g) green apple flavoring
FOR THE APPLE COULIS
4 Granny Smith apples, peeled, cored, cut into chunks
3 Tbsp (50 g) unsalted butter
1/4 cup (60 g) sugar
1 tsp (5 g) cinnamon
1 tsp (5 ml) fresh lemon juice
Pinch of salt
Water, as needed
FOR THE BISCUIT
5 egg yolks
4 oz (30 g) xylitol sweetener
6 egg whites
1/2 cup (55 g) all-purpose flour
1/4 cup (60 g) potato starch
Make the soaking syrup: Cook the sweetener and water together until thickened slightly; add the apple flavoring. Allow to cool.
Make the apple coulis: Cook the apples, butter, sugar, cinnamon, salt, and lemon juice, covered, over low heat until the apples are very tender. Puree the apple mixture until very smooth, adding a bit of water if necessary. Press through a fine-mesh sieve. Set aside to cool.
Make the biscuit: Preheat the oven to 350ºF (180ºC).
Using an electric mixer, beat the egg yolks with 1 oz sweetener. In a separate bowl, beat the egg whites until foamy; add 3 oz sweetener and beat until stiff peaks form. Gently fold the whites into the yolks. In a separate bowl, sift the flour and potato starch together and fold them into the egg batter. Spread on a greased large baking sheet and bake for about 8 minutes.
Make the diplomat cream: Put the milk and 3/4 cup cream into a saucepan; add the vanilla beans. Heat the mixture just until bubbles begin to form; remove from heat. Leave it to infuse, 30 minutes.
Beat the egg yolks with the sweetener until the mixture grows pale. Beat in the cornstarch. Add this mixture little by little into the warm milk and cook over low heat, stirring constantly. When the cream thickens, add the gelatin and cook 2 minutes, stirring; remove from heat. Strain through a fine-mesh strainer, then blend with an immersion blender. Cool to room temperature.