1. Prepare the Crème Pâtissière: In a saucepan, bring milk and vanilla bean (or extract) to a simmer. Remove from heat and let it infuse for 15 minutes. Remove the vanilla bean.

2. In a bowl, whisk egg yolks and sugar until pale and mousse-like. Add cornstarch and mix until smooth. Gradually pour warm milk into the egg mixture, whisking constantly. Return to the saucepan and cook over medium heat, stirring constantly until thickened.

3. Remove from heat and add 100g of softened butter. Mix vigorously until fully incorporated. Pour into a dish, cover directly and let cool completely in the refrigerator.

4. Prepare the Crème Mousseline: Once the pastry cream is completely cool, beat it with an electric mixer until smooth and light. Gradually add the remaining 100g of softened butter, continuing to beat until creamy and smooth.

5. Final Assembly: In a cold bowl, beat the very cold heavy cream into firm whipped cream. Gently incorporate the whipped cream into the cooled crème mousseline using a spatula.