In a large mixing bowl, add softened cream cheese and mix on low speed until smooth.
Gradually add sweetened condensed milk and blend until well combined.
Stir in freshly squeezed lemon juice and lemon zest.
In a separate bowl, whip cold heavy cream with vanilla extract until stiff peaks form.
Gently fold the whipped cream into the cream cheese mixture until fully incorporated and smooth.
In a small bowl, mix graham cracker crumbs with melted butter to form a cohesive crust.
Spoon a small amount of graham cracker mixture into the bottom of each mousse cup liner or individual serving glass.
Carefully spoon the lemon cheesecake mixture over the crust and smooth the top.
Cover and refrigerate for at least 2 hours until firm.
Just before serving, garnish with additional lemon zest or a dollop of whipped cream if desired.