Prepare a square 8x8 baking pan with parchment paper and set it aside.
Add the plums, sugar, brown sugar, and chai spice to a pot. Cook over medium-high heat for 10-12 minutes or until they release all their juices. Use a masher to mash them. Continue cooking for another 8 minutes, or until the jam is thick.
3 cups chopped red plums, ¼ cup granulated sugar, 2 tablespoon light brown sugar, 1 ½ teaspoon chai spice
Whisk together the cornstarch and water and pour it into the pot.
2 ½ tablespoon cornstarch, ⅛ cup water
Cook for another 5 minutes, or until the jam is even thicker and paste-like.
Transfer the jam into a small bowl and place it in the fridge to chill.
When the jam is completely cool, make the shortbread.
In a stand mixing bowl combine the butter, flour, sugar, brown sugar, and salt on low speed with the paddle attachment until the dough is crumbly and in pea sized pieces, this can take anywhere from 3-6 minutes.
2 cups all-purpose flour, 1 cup unsalted butter, ⅓ cup granulated sugar, 2 tablespoon light brown sugar, ¾ teaspoon vanilla extract, ½ teaspoon kosher salt
Preheat the oven to 350 F/180 C.
Reserve ½ cup of the shortbread dough and set it aside.
Evenly press the rest of the shortbread into the prepared baking pan.
Evenly smear the cooled jam on top of the shortbread.
Sprinkle the reserved shortbread crumbs on top of the jam.
Bake for 33-38 minutes, or until the top of the shortbread is starting to turn golden brown.
Let the bars cool completely on a wire rack.
When cool, place the pan in the freezer for 15-20 minute to harden. Then remove them from the pan and use a sharp knife to cut them into clean bars. Dust with powdered sugar and enjoy!