Make the Vanilla Mousse:

• Bloom Gelatin: Combine gelatin and cold water in a small bowl. Stir and let sit for 5 minutes.

• Prepare Cream Mixture: In a saucepan, heat milk, heavy cream, and 1/4 cup sugar over medium heat until hot, but not boiling. Remove from heat.

• Whisk Egg Yolks: In another bowl, whisk egg yolks and remaining 2 tablespoons of sugar until pale and smooth.

• Temper the Eggs: Gradually whisk the hot cream mixture into the egg yolks. Return mixture to the saucepan and cook on low heat, stirring until it thickens to coat the back of a spoon (170°F/77°C).

• Add Gelatin: Stir in the bloomed gelatin and vanilla extract until fully dissolved. Let the custard cool at room temperature, then refrigerate for 30 minutes.

• Whip Cream: In a separate bowl, whip the heavy cream until soft peaks form. Gently fold into the chilled custard.

Make the Chocolate Layer:

• Melt Chocolate: Heat heavy cream in a saucepan until simmering, then pour over chopped chocolate. Let sit for 2-3 minutes before stirring until smooth.

• Add Butter & Vanilla: Stir in butter (if using) and vanilla extract. Let the mixture cool to room temperature.

Assemble the Dessert:

• Spoon the vanilla mousse into serving glasses. Top with the cooled chocolate layer.

• Refrigerate for 2-3 hours until fully set.

Serve: Optionally, garnish with grated chocolate, fresh berries, or whipped cream.