Line a 9x13-inch pan with parchment, leaving overhang for easy lifting.
Beat cream cheese in a large bowl until perfectly smooth and fluffy.
Stream in the condensed milk and mix until silky with no lumps.
Whisk in lemon juice, lemon zest, vanilla, and salt until the mixture thickens slightly.
Whip the cold cream with powdered sugar to soft, billowy peaks.
Fold the whipped cream gently into the lemon mixture to make a light mousse.
Arrange a snug layer of graham crackers in the pan. Spread on a thick layer of lemon mousse.
Repeat layers, ending with mousse on top. Smooth the surface and sprinkle graham crumbs over.
Cover and refrigerate until the crackers soften and the cake sets, at least 4 hours or overnight.