The filling for this pretty pink tart is based on this blackberry panna cotta recipe. The tart shell is my go-to sweet shortcrust pastry from the cookbook Soulful Baker, by Julie Jones.
Place the flour and butter in the bowl of a stand mixer fitted with a paddle attachment. Beat on low speed until the mixture resembles fine breadcrumbs.
Add the powdered sugar to the bowl, and beat gently until just combined.
Add the egg yolk and milk, turn the speed up to medium-low (2 or 3 on a KitchenAid stand mixer), and mix until the dough just starts to come together.
Gather the dough mixture in a piece of plastic wrap, bringing it together into a ball, and then flattening the ball into a disk about 1/2 inch thick.
Chill the pastry dough in the refrigerator for at least one hour.
When the dough is chilled, roll it out between two pieces of parchment paper (this will allow you to avoid adding more flour) until it is about 1/8 inch thick.
Remove the top sheet of parchment and transfer the dough (parchment side up) to a 9-inch fluted tart pan. Remove the remaining parchment and gently press the dough into the pan. Trim the edges so that the pastry extends about 1/2 inch beyond the top of the pan. (You want a bit of overhang.) Use the handle of a wooden spoon to gently press the the pastry into the fluted sides of the pan. Reserve the extra dough.
Place the tart shell in the refrigerator and chill for about 30 minutes.
Meanwhile, preheat the oven to 400 F.
Remove the tart shell from the refrigerator, place it on a baking sheet, and prick the bottom all over with a fork at 1 inch intervals. Line the pastry with a crumpled up piece of parchment paper, and fill the shell with pie weights, dried beans, or rice.
Blind bake the tart shell for 15 minutes. Then take the baking sheet with tart shell out of the oven, carefully remove the parchment and pie weights, and return to the oven to bake for an additional five minutes, until the dough loses its "raw" look.
Remove the pan from the oven again. If there are any cracks or holes, patch them with the reserved dough. Mix the remaining egg yolk with a couple of drops of boiling water, and brush the inside of the tart shell with the egg yolk mixture.
Return the tart shell to the oven and bake for another 15 minutes.
Remove the tart shell from the oven to a wire rack and allow it to cool, still on the baking sheet.
When the tart shell is cool, use a Swiss vegetable peeler to carefully shave away the overhang. Using a clean, soft, pastry brush or paintbrush, gently sweep out any crumbs that have fallen into the bottom of the shell.
In a saucepan, combine the chopped rhubarb and the sugar and a couple of teaspoons of water, bring to simmer, and cook over low to medium-low heat until rhubarb is almost completely broken down. Set aside to cool for 10-15 minutes.
Add the cream to the rhubarb-sugar mixture and heat until just steaming (160-170 F). Remove from the heat, and allow the mixture to steep for 30 minutes.
Toward the end of the 30 minutes, place the 3.5 tbsp cold water in large, spouted container. (I used a 4-cup capacity Pyrex measuring cup.) Sprinkle the powdered gelatin over the cold water.
Return the rhubarb-cream mixture to the stove, and re-heat again until just steaming (160-170 F). Stir in the vanilla bean paste.
Place a fine mesh sieve over the container with the now softened gelatin, and pour in the rhubarb-cream mixture through the sieve, using a rubber or silicone spatula to stir and press mixture to extract every last bit of pink-tinged liquid. When you've extracted as much liquid as possible, thoroughly whisk the currant-cream mixture into the softened gelatin.
Place the tart shell, still on the baking sheet, in the refrigerator on a low shelf.
Carefully pour the filling into the tart shell, then leave the tart in the refrigerator to set overnight, or for at least four hours.
When the panna cotta filling has fully set, decorate the tart as desired with rhubarb ribbons or rhubarb roses.
Instructions for making rhubarb roses are here.
Rhubarb ribbons instructions can be found here.
One person who made this recipe commented that her filling set with lots of unsightly little bubbles on the surface. To avoid this result, try not to whisk in too much air when combining the rhubarb-cream with the softened gelatin, and pop any stray bubbles with a toothpick after you pour the filling into the shell.
The rhubarb flavor in this tart is definite but subtle. For more intense rhubarb flavor and pink color, try increasing the amount of rhubarb.