Heat syrup and 3/4 cup milk in a saucepan over medium heat until simmering.
Combine cornstarch, salt, and remaining milk. Add to simmering syrup mixture, stirring constantly.
Cook over medium-high heat, stirring constantly, until mixture is thickened and begins to bubble.
Reduce heat to medium; cook and stir 2 minutes.
Beat egg in a small bowl. Add in a small amount of the hot mixture and stir well.
Add egg mixture to pan and cook, stirring constantly, until mixture simmers. It will be thick.
Continue to cook and stir for 2 additional minutes.
Pour into a heat-proof bowl. Cover with plastic wrap or waxed paper, making sure wrap touches the surface of the custard to prevent a skin, and refrigerate until cool, 4 hours or overnight.
When custard is cool, whip cream until stiff peaks form.
Gently fold into cooled custard, then transfer to serving bowls. Top as desired and serve.
Recipe Notes
Candied or roasted walnuts would be a great addition to the top of this mousse.
Use mousse as a filling for cannoli or cream puffs.
For a firmer mousse, use a stabilized whipped cream (with gelatin).
