zille, when the yolk mixture is done it should be 165-170F with an instant read thermometer AND be tripled in volume. The texture should be light and whipped, a thick egg foam rather than a thin, liquid custard.
I imagine it might be difficult to use an electric mixer with a double boiler. If you have an immersion blender that might work if it has a slower setting. The problem with a mixer/blender is that it whips the egg mixture so fast that it doesn’t heat up to the right temperature. When hand whisking, go slow enough that you still see the temp rising.
Yes, you can use whipping cream, which is usually heavy cream with emulsifiers added.