Place raspberries in a saucepan with granulated sugar and lemon juice. Stir and cook over medium heat until the berries release their juices and mixture comes to a boil.
In a small bowl, mix the cornstarch and water to make a slurry. Stir into raspberries. Return to a boil and cook until mixture thickens. This will happen quickly. Remove from heat.
Set a sieve over a small mixing bowl and pour raspberry mixture into the sieve. Using the back of spoon, push raspberry liquid through sieve into the bowl beneath. Seeds will be trapped on top. Keep working until no more liquid drips into the bowl. Discard seeds. Set aside.
MAKE THE WHIPPED CREAM
In the bowl of an electric mixer fitted with the whisk attachment, combine the heavy cream, powdered sugar, 1 tsp lemon zest and vanilla. Mix on low speed to combine and then slowly raise the speed to high, until stiff peak stage.
ASSEMBLE THE CAKE
Arrange shortbread cookies flat in an 6-inch springform pan, covering the bottom and breaking the cookies if needed to fill the space. *OR loaf pan lined with plastic wrap to overhang.
Spread 1/3 of the whipped cream evenly over the cookies. Gently spread 1/2 raspberry puree over whipped cream. Place another layer of cookies on top, lying flat and touching, followed by another 1/3 of the whipped cream then the remaining raspberry puree. Follow with layer of remaining cookies and whipped cream..
Smooth the top, cover with plastic wrap, and refrigerate 8 hours or overnight.
Run a small sharp knife around the outside of the cake and remove the sides of the springform pan. Leave the cake on the pan bottom or remove with a spatula. Garnish with remaining lemon zest and reserved raspberry puree.