Crumb cookies into fine crumbs and combine with melted butter. Press firmly into a lined cake pan to create a solid crust.
Beat cold heavy cream until it forms soft peaks. This will provide the mousse with a light, fluffy texture without over-stiffening it.
Melt chocolate slowly with a small amount of cream over low heat or in brief bursts in the microwave. Stir until it's smooth and lump-free.
Gently fold whipped cream into cooled chocolate. Move gently to preserve fluffiness in the mixture and prevent deflating.
Spread the mousse over the crust, smooth the surface, and chill for at least 4 hours. A good chill secures the silky texture.