Bring 1 inch of water to a boil in a 4-quart saucepan set over high heat.
Add the egg yolks and sugar to a large glass bowl. Using an electric hand mixer on the highest speed, beat the eggs and sugar until the mixture is thick, pale yellow, and the sugar is completely dissolved, 4 to 5 minutes. Decrease the mixer speed to low and add the Marsala and salt.
Decrease the heat to maintain a simmer and place the bowl atop the saucepan with the boiling water, making sure the bottom of the bowl is not touching the water. Beat the mixture on medium speed until it is thick, frothy, holds a ribbon, and the temperature reaches 145 to 150ºF on a digital instant-read thermometer.
Spoon the warm mixture into custard cups or serving glasses and serve immediately or allow to cool slightly. Serve with fresh berries, if desired.