Begin by making the curd. In a small sauce pan, whisk together the lemon zest, lemon juice, sugar, and egg. Cook over low heat, whisking continually, until slightly thickened. Then, add the butter a piece at a time while constantly whisking. Wait until one piece of butter is melted before adding another. Continue whisking until the curd is smooth and thickened. (It should coat the back of a spoon.)
Strain with a fine mesh sieve by pushing the curd through the sieve with a spatula. Transfer to a bowl and press a piece of plastic wrap directly onto the surface of the curd. Chill until cold.
In the meantime, prepare your tart crust. Begin by preheating your oven to 350 degrees F. Spray 6 (3-inch) tart pans with baking spray. Set aside. In a small bowl, whisk together the flour, salt, and cinnamon. In a medium bowl, combine the melted butter, sugar, and vanilla. Add the flour mixture and fold until incorporated. Divide the batter evenly among the tart pans.
Using your fingers, press the dough into the bottom and sides of each pan. Pierce the dough with a fork. Bake the crust for about 10 minutes or until set. Transfer to a wire rack to cool.
For the filling, add the cream cheese, Greek yogurt, and sugar to a bowl. Mix with an electric mixer until smooth. Then add the salt, vanilla, and egg. Mix again until smooth. Spoon the cream cheese mixture into the tart pans until each is about 4/5 full.
Add the lemon curd to a plastic piping bag (or plastic baggie with the corner cut off) and pipe small dots onto the cream cheese filling. Swirl together with a toothpick or wood skewer. Gently tap tart pans to remove any air bubbles.
Bake for 25-30 minutes or until the center is set. Remove from the oven and cool completely on a wire rack. Cover and chill in the refrigerator until you're ready to serve. Garnish with a dollop of whipped cream* and a slice of lemon.