In a large mixing bowl, combine the sweetened condensed milk and fresh lime juice. Stir until well mixed and set aside to allow the mixture to thicken slightly, about 5 minutes.
In another bowl, whip the heavy cream and vanilla extract until stiff peaks form.
Gently fold the whipped cream into the lime mixture until fully combined, being careful not to deflate the whipped cream.
In a rectangular dish, arrange a layer of Maria cookies at the bottom. Pour a layer of the lime mixture over the cookies, spreading it evenly.
Repeat the layers, alternating between cookies and lime mixture, until all ingredients are used, finishing with a layer of the lime mixture on top.
Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight for best results.
Before serving, garnish with lime zest if desired. Cut into squares and enjoy!