1. Prepare the Egg Yolk Mixture:

• In a medium bowl, whisk together the egg yolks and 1/4 cup of the granulated sugar until the sugar is dissolved and the mixture is lightened in color, about 1 minute. The mixture should be pale yellow and slightly thickened.

2. Simmer the Cream:

• In a small pot, combine 1 cup of the heavy cream and the salt.
• Bring just to a simmer over medium heat. Watch carefully – you want to see small bubbles forming around the edges of the pot, but do not bring it to a full boil.
• Once simmering, remove from the heat.

3. Temper the Egg Yolks:

• Remove 1/4 cup of the warm cream mixture from the pot.
• Slowly drizzle the warm cream into the egg yolk mixture, whisking constantly. This process is called tempering, and it gently warms the egg yolks, preventing them from scrambling when added to the hot cream.

4. Cook the Custard Base:

• Transfer the warmed egg yolk mixture back to the pot with the remaining heavy cream.
• Whisk well to combine.
• Return the pot to medium-low heat.
• Stir frequently with a rubber spatula, making sure to scrape the bottom and corners of the pan to prevent sticking and scorching.
• Cook until the mixture thickens just enough to coat the back of the spatula. This should take a few minutes. The temperature should be between 160°F and 180°F (71°C and 82°C). Use a thermometer for accuracy.

5. Melt the Chocolate:

• Remove the pot from the heat.
• Stir in the chopped chocolate and espresso powder (if using) until completely melted and smooth. The residual heat from the custard base will melt the chocolate.

6. Cool the Chocolate Mixture:

• Transfer the chocolate mixture to a large bowl.
• Refrigerate for 20 minutes, stirring halfway through, until slightly cooled. This prevents the chocolate from seizing up when you fold in the whipped cream. Do not refrigerate for too long, or the chocolate will harden.

7. Whip the Cream:

• In another large mixing bowl, combine the remaining 1 1/2 cups of heavy cream, the remaining 2 tablespoons of sugar, and the vanilla extract (if using).
• Using an electric handheld mixer, whip the cream until it forms soft peaks. Soft peaks are when the cream holds its shape but the peaks curl over slightly. Be careful not to overwhip.

8. Fold in the Whipped Cream:

• Reserve 1/2 cup of the whipped cream in the refrigerator for serving.

• Stir a large dollop (about 1/4) of the whipped cream into the cooled chocolate mixture until thoroughly combined. This lightens the chocolate mixture and makes it easier to fold in the remaining whipped cream.

• Add the remaining whipped cream to the chocolate mixture.

• Using a rubber spatula, gently fold the whipped cream into the chocolate mixture. Use a scooping motion, bringing the chocolate mixture from the bottom of the bowl up and over the whipped cream.

• Continue folding, rotating the bowl, until the whipped cream and chocolate are thoroughly combined, and there are no streaks of white remaining. Be gentle to avoid deflating the whipped cream.

9. Chill and Serve:

• Divide the mousse mixture evenly among 6 (6-ounce) ramekins or dessert glasses.
• Serve immediately, or refrigerate for up to 1 hour to allow the mousse to set further. Chilling slightly firms up the texture.
• Top with the reserved whipped cream, chocolate shavings, and fresh raspberries before serving, if desired.