In a medium size bowl, stir together melted butter, graham crackers and granulated sugar until completely moist. Press into a 14-inch tart pan and set aside.
In a large bowl, stir together cream cheese and sugar until smooth. Scrape sides of bowl and mix again briefly. Add in sweetened condensed milk while mixer is on, then drop eggs in one at a time until combined. Scrape sides and stir again. Finally add in lemon juice and zest. Scrape down bowl to ensure batter is smooth and evenly mixed and pour overtop crust. Smooth the top and bake 20-25 minutes until set. Remove from oven and cool to room temperature. Then cover with plastic wrap and refrigerate until chilled. Add sweetened whipped cream and lemon zest as a garnish if desired and serve.