Line the bottom of a 9-inch springform pan with parchment paper.
Combine the cookie crumbs, melted butter, and granulated sugar. Mix until the crumbs are evenly moistened.
Press the mixture firmly into the bottom of the prepared pan. Refrigerate while preparing the filling.
Add the chopped strawberries, half of the granulated sugar, and lemon juice to a blender or food processor. Blend until smooth. Strain the puree if you prefer a smoother mousse.
Place the cold water in a small bowl and sprinkle the gelatin over it. Let it stand for about 5 minutes to bloom
Warm the strawberry puree gently in a small saucepan. Do not bring it to a hard boil.
Add the bloomed gelatin and stir until completely dissolved. Remove from the heat and let the strawberry mixture cool until it is no longer warm.
Beat the cold heavy cream with the remaining sugar and vanilla until medium to firm peaks form.
Gently fold the whipped cream into the cooled strawberry mixture in batches. Fold slowly to preserve the airy texture.
Spoon the mousse over the chilled cookie crust and smooth the surface.
Cover and refrigerate for at least 6 hours, preferably overnight, until the mousse is firm.
Before serving, whip the remaining heavy cream with powdered sugar and vanilla until firm peaks form.
Carefully remove the sides of the springform pan. Decorate the cake with whipped cream and fresh strawberries.
Slice with a sharp knife warmed under hot water and dried thoroughly. Serve chilled.
