Using a paring knife, trim tip end from a lemon so that fruit sits level. Cut stem end 1/4 horizontally down the lemon (or cut in a decorative pattern). Repeat process with 7 lemons.
Hold a cut and trimmed lemon over a sieve set over a liquid-measuring cup. Using a reamer, squeeze juice, and set aside; then scoop out flesh with a spoon (do not puncture lemon skin), and discard. Repeat with remaining cut lemons. Measure 1/4 cup juice, and set aside. Reserve remaining juice for another use. Cover lemon shells with plastic wrap, and refrigerate. With a grater, remove zest from 1/2 of the remaining lemon; cover zest with plastic, and set aside.
Place egg yolks, 1/4 cup lemon juice, and 6 tablespoons sugar in a small heavy saucepan. Whisk to combine. Set over medium heat; stir constantly with a wooden spoon, making sure to stir sides and edges of the saucepan. Cook until mixture is thick enough to coat the back of a wooden spoon, 3 to 5 minutes.
Remove saucepan from heat. Pass mixture through a fine sieve set over a medium bowl. Add butter, 1 piece at a time, stirring with a wooden spoon until smooth. Stir in reserved lemon zest. Place plastic wrap directly on the surface to prevent a skin from forming, and chill until firm, at least 1 hour.
In a medium bowl with a handheld electric mixer, whip cream to stiff peaks. Cover with plastic wrap, and refrigerate. Clean the whisk attachments. In a second medium bowl, with a handheld electric mixer, whip egg whites and a pinch of salt to soft peaks. Sprinkle in remaining 2 tablespoons sugar, and continue whipping until stiff glossy peaks form. Set aside.
Remove lemon curd and whipped cream from refrigerator. Stir 1/4 of whipped cream into curd to lighten the curd. With a large rubber spatula, gently but thoroughly fold in remaining whipped cream. Fold in whipped egg whites.
Remove lemon shells from refrigerator. Transfer mousse to a medium pastry bag fitted with a coupler, and pipe into lemon shells. Chill until set, a minimum of 1 hour or overnight.
Note: Raw eggs should not be used in food prepared for pregnant women, babies, young children, the elderly or anyone whose health is compromised.
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