First, place the bowl you plan to use for the whipped cream in the refrigerator so it becomes nicely chilled. Fifteen to thirty minutes is usually enough. This step is not strictly necessary, but a cold bowl helps the cream whip up firmer and faster.

Once the bowl is cold, pour in the heavy whipping cream and add the two tablespoons of sugar. Whip the mixture until it becomes thick and fluffy. You may do this with a whisk by hand if you prefer, though it takes a little patience, or use a hand mixer or stand mixer for convenience. Beat just until stiff peaks form. In a stand mixer this usually takes about two to three minutes. Be careful not to whip too long. When the cream is ready, place it in the refrigerator to keep it cold until needed.

Next, prepare a gentle double boiler. Fill a saucepan with cold water and bring it to a light simmer over the stove. Choose a bowl that can sit comfortably over the pot without touching the water beneath.

In that bowl, combine the egg yolks, whole eggs, and sugar. Set the bowl over the simmering water and whisk steadily. Continue whisking for about eight to ten minutes, until the mixture thickens and begins to resemble a smooth custard.

Add the lemon zest, fresh lemon juice, and a small pinch of salt. Keep the bowl over low heat and stir slowly with a spatula as the mixture cooks down. It should thicken further and take on a glossy texture, almost like a soft jelly, yet still creamy.

Remove the bowl from the heat and stir in the butter while the mixture is still warm. The butter will melt smoothly into the lemon custard. Cover the surface tightly with plastic wrap and place it in the refrigerator to cool for about twenty to thirty minutes.

Once the lemon mixture is chilled, remove it along with the whipped cream from the refrigerator. Set aside about half a cup of the whipped cream for finishing later. Using a spatula, gently fold the remaining whipped cream into the lemon mixture. Work slowly, folding in soft figure eight motions until the color is uniform and no streaks remain. Take care not to overmix so the mousse stays light and airy.

Spoon the finished mousse into serving cups or small ramekins. Smaller dishes will make about four portions, while larger ones will make two generous servings. Place them in the refrigerator and allow the mousse to chill for at least two hours, or overnight for the best texture.

When ready to serve, top each portion with the reserved whipped cream. A few thin slices of lemon, a little extra zest, or fresh berries make a lovely finishing touch.