Boil milk, first quantity of cream, and vanilla bean seeds.
Whisk yolks with sugar, inulin, and custard powder until light.
Bloom gelatin in cold water for 10 minutes and melt gently.
Temper yolk mixture with hot cream, then return all to pan and bring to boil. Add gelatin and mix until smooth.
Add second quantity of cold cream and emulsify with hand blender.
Store in vacuum bags, cool rapidly to 4°C (39.2°F), freeze 20 minutes before use.
Whip when ready to use to a creamy texture at 0–2°C (32–35.6°F).
🧈 Optional mousseline: Add 200 g whipped butter per 1000 g of whipped cream with 5–10% flavoring paste (e.g., pistachio).