Line the base of an 8-inch springform pan with parchment paper. Combine the cookie crumbs, melted butter, optional sugar, and salt, then press the mixture firmly into the pan and refrigerate it while preparing the mousse.

Sprinkle the gelatin over the cold water in a small bowl and let it stand for 5 minutes. Melt it gently in short microwave intervals or over hot water, taking care not to boil it.

Beat the softened cream cheese, confectioners' sugar, lemon zest, lemon juice, vanilla, and salt until completely smooth. Stir a small amount of this mixture into the melted gelatin, then quickly blend that mixture back into the remaining lemon mixture.

Whip the cold heavy cream to medium-stiff peaks, stopping before it becomes grainy or buttery. Fold the whipped cream into the lemon mixture in two or three additions to keep the mousse airy.

Spoon the mousse over the chilled crust and smooth the surface. Refrigerate the cake for at least 4 hours, or overnight when possible, until firm.

Run a warm, thin knife around the edge before releasing the springform ring. Garnish just before serving with lemon slices, lemon zest, whipped cream, or a dusting of confectioners' sugar.