1. Grease a 9 inch tart pan and preheat oven to 350 degrees.
2. Combine graham cracker crumbs, sugar and melted butter.
3. Press graham cracker mixture into the bottom and up the sides of the tart pan.
4. Bake for 7 minutes, then remove from oven and set aside.
5. Whisk together the lemon juice and sweetened condensed milk. Set aside.
6. In a mixer bowl, beat the lemon zest and egg yolks on high until pale.
7. Add sweetened condensed milk mixture to egg mixture and beat until smooth.
8. Pour filling into the crust.
9. Bake for 15-17 minutes. The edges should be set and the center should jiggle a little when the tart is moved.
10. Cool for 30 minutes, then refrigerate for at least 6 hours, or until firm.
11. To make whipped cream, whip heavy whipping cream until it begins to thicken.
12. Add powdered sugar and vanilla extract and whip until stiff peaks form.
13. Add whipped cream to a piping bag fitted with the Ateco 844 tip, or similar (Wilton 1M will work). Pipe whipped cream in circles, starting from the center and working outward. Fill in gaps with drop flowers by piping little drops of whipped cream (they look like small flowers). Refrigerate until ready to serve.