In a large heat-proof bowl, whisk together 3 whole eggs, 3 egg yolks (reserve the whites in a separate bowl), 1 cup of the granulated sugar, lemon zest, lemon juice, and salt. Set aside.
Fill a large saucepan with a few inches of water and bring to boiling. Reduce to a simmer.
Place the bowl of ingredients over the pan of simmering water and cook, stirring constantly with a wooden spoon, for about 15 minutes, or until mixture starts to thicken. I use my digital instant-read thermometer to check the temperature has reached 150ºF (66ºC).
Remove bowl from heat and set aside for 15 minutes. Place a piece of plastic wrap directly on the surface of the mixture and refrigerate for at least 2 hours, or until completely chilled.
Place the 3 remaining egg whites into a medium size mixing bowl or the bowl of a stand mixer fitted with the whisk attachment and beat on high speed for 1 minute. Add the remaining 2 Tablespoons of granulated sugar and continue to beat until the egg whites reach stiff peaks.
Carefully fold the beaten egg whites into the cold lemon mixture (don't get rid of the bowl), going slowly and being careful not to deflate the egg whites. Set aside.
Add the heavy cream to the bowl you used for the egg whites, and beat on high speed until stiff peaks form.
Fold the stiff whipped cream into the lemon mixture, again going slowly and being careful not to deflate the mixture. Optional: if you'd like to stretch your lemon mousse further and/or amp up the lemon flavor even more, gently fold in homemade lemon curd.
Spoon the lemon mousse into individual parfait cups or leave in one bowl (to be scooped out per serving) and chill for at least 30 minutes. Top with homemade whipped cream2 and/or homemade lemon curd and serve cold. Store in the refrigerator, covered tightly, for up to 5 days.
Notes
Eggs: keep three eggs whole, and separate three other eggs. The three yolks will go in with the three whole eggs. Reserve the three egg whites in a separate bowl.
Homemade whipped cream: one pint of heavy whipping cream will be enough for this recipe + a half batch of my homemade whipped cream for topping.
All nutritional values are approximate and provided to the reader as a courtesy. Changing ingredients and/or quantities will alter the estimated nutritional calculations.