Cut rhubarb stalks into 4.5-inch wide segments. Test length by placing cut stalks in empty tart pan - stalks should fit easily in pan with space on all sides for crust edges.
Add maple syrup, water, and vanilla extract to large skillet over medium heat. Gently whisk liquids until mixture begins to simmer, then add cut rhubarb stalks.
Reduce heat to medium-low and simmer 5 minutes. After 5 minutes, remove skillet from heat and cover with lid. Let topping cool in covered pan at least 1 hour (overnight preferred).
For the Shortbread Crust
Preheat oven to 350° Fahrenheit.
Add all crust ingredients to bowl of food processor. Pulse ingredients 3 to 5 times or until mixture is combined and holds together when pinched.
Transfer crust mixture to tart pan. Gently but firmly press crust into bottom of pan and up sides of pan in one even layer.
Place tart pan in preheated oven. Bake 11 to 13 minutes or until sides of crust are lightly brown. Remove crust from oven and set aside to cool.
For the Frangipane Filling
Wipe out food processor bowl to remove any leftover crust crumbs. Add all frangipane ingredients to food processor bowl, then pulse 2 to 4 times, until mixture is well-combined.
Transfer frangipane filling to tart pan with partially-baked tart crust. Use spatula to evenly distribute filling across crust and smooth top of filling layer.
Cut one large sheet of aluminum foil and place filled tart pan in center of foil sheet. Bring sides of foil up, folding as needed so that foil just covers crust tart edges while leaving filling uncovered.
Return tart pan to preheated oven. Bake 20 minutes or until filling is golden-brown and set. Check doneness by inserting knife in center of filling and removing - knife should come out of filling completely clean.
When filling is set, remove tart pan from oven and set aside to cool completely.
For the Rhubarb Tart
Strain rhubarb out of syrup and set syrup aside. One by one, gently line rhubarb across top of frangipane filling.
Return skillet with syrup to stovetop and warm uncovered over medium heat. Let syrup come to boil, then continue boiling until syrup has reduced to approximately ¼ cup.
Use pastry brush to gently brush reduced syrup over rhubarb topping, then serve tart immediately or cover pan with foil and keep at room temperature until ready to serve.
If you don't have a rectangle tart pan, you could try making this in a 9x9" baking dish. You'll need a little more rhubarb, and the filling and crust will be a bit thinner, but it would otherwise be a close substitution. I highly recommend getting a rectangle tart pan, though - so elegant and stunning (plus quite affordable)!
Serving: This tart is best served immediately. If you plan to serve it the same day you make it but not immediately after baking, cover the pan with foil and keep the tart at room temperature until ready to serve. If you plan to serve it the next day, you'll need to refrigerate the tart in an airtight container until ready to serve.