Sprinkle gelatin over water; let stand until softened, about 5 minutes.
Whisk together eggs and yolks in a heavy small saucepan. Whisk in sugar, lemon zest, and lemon juice. Cook, whisking constantly, over medium-low heat, until mixture is thick enough to coat the back of a wooden spoon, 8 to 10 minutes.
Remove pan from heat; add gelatin, stirring constantly, until gelatin dissolves and mixture is slightly cool. Add butter, a few pieces at a time, stirring after each addition, until smooth.
Strain through a fine sieve into a bowl, pressing with a rubber spatula to remove as much curd as possible; discard any undissolved bits of gelatin and egg. Press a piece of plastic wrap directly onto surface of curd to prevent a skin from forming, and refrigerate until set, at least 2 hours or overnight. Stir curd, and gently fold in whipped cream. Refrigerate for 1 hour. Stir before using.