In a medium-sized mixing bowl, beat butter until light and fluffy.
Beat in brown sugar until fully combined.
Mix in flour, salt, and cinnamon until just combined.
Press dough into the bottom and up the sides of a greased 11" tart pan with a removable bottom. Be sure the dough goes all the way to the top of the sides of the pan. It may fall a little during the baking and if it's already shy of the top, it will be even lower after the bake.
Use a fork to poke shallow holes all over the dough.
Bake for 20 minutes.
Port Syrup Instructions
In a small, heavy-bottomed saucepan, bring port, brown sugar, and cinnamon stick to a boil.
Lower heat to a simmer and cook until liquid reduces to 2/3 cup, 10-15 minutes. It's totally OK to pour the liquid out into a glass measuring cup to check the amount and then return it to heat if necessary. I did this a couple of times. :)
Remove the cinnamon stick. I like to suck on them, but toss them if you must.
Pluot Filling, Assembly, and Baking Instructions
Up the oven heat to 350 F.
Slice pluots in half, remove their pits, and cut each half into 8 thin slices.
Coat pluot slices in flour and mix in 1/3 cup of the syrup.
Arrange the pluot slices in a spiral (or any pattern you prefer) over the pre-baked crust.
Pour any syrup left in the bottom of the pluot bowl and the remaining 1/3 cup of syrup evenly over the tart.
Bake for 50 minutes or until the crust is golden brown and the syrup is bubbling and looks thicker around the edges of the tart. There will still be plenty of liquid; it will come together as it cools.
Cool on the counter until room temperature, then transfer to the refrigerator to chill for at least two hours.
Once cool, remove the outer ring of the pan (the easiest way to do this is to set the pan on top of an upside-down bowl), slice, and serve.
This tart can also work with plums, peaches, or nectarines. If you cut the tart before it cools, the cut won't be as clean. Juice will spill all over. It will still taste good, but expect a delicious mess.