Preheat oven to 350 degrees.
To make the vanilla shortbread crust, blend all ingredients together in a food processor until it just begins to clump together. Press evenly into a tart pan with a removable bottom. Bake for approximately 15-20 minutes, turning halfway through until just starting to brown. Remove from oven and cool.
In a medium saucepan over medium heat, cook the rhubarb, raspberries, 1/4 cup sugar and water, stirring frequently, until sugar is dissolved and rhubarb and raspberries are soft and mushy. Cook for another couple of minutes then remove from heat. Put the mixture through a sieve, pressing on the solids to extract all the liquid. Set the liquid aside and discard the solids.
In the bowl of a double boiler over simmering water, add egg yolks, scant 1/2 cup sugar, lemon juice, lemon zest and butter. Whisk until sugar is dissolved, and continue whisking over simmering water until the egg mixture begins to thicken. Slowly add the rhubarb/raspberry liquid and continue stirring, about 5-7 minutes, until the mixture thickens to a thick, pudding like consistency. Remove from heat, cool and store in a clean, sterilized jar until ready to use.
To assemble the tart: Spread rhubarb raspberry curd onto cooled tart shell and chill for at least 30 minutes. Top with vanilla whipped cream and serve.