Chop the plums into chunks and remove the pits. Add them to a small pot along with the sugar and chai spice. Cook over medium high heat for 10 minutes, then mash them with a wooden spoon.
3 ripe black plums, 1 ½ teaspoon chai spice, ¼ cup white sugar
Whisk together the corn starch and water in a bowl and pour it into the pot. Let the jam cook and reduce down for 5 minutes, or until the jam is super thick and paste like.
1 ½ tablespoon corn starch, ⅛ cup water
Remove the jam from the heat and add it to a bowl. Set aside and let cool completely before using.
Next, cut the cold butter into cubes. Add the flour, salt, sugars, butter cubes and vanilla to a stand mixing bowl.
1 ½ cups all purpose flour, ½ teaspoon kosher salt, 1 ½ sticks unsalted butter, ¼ cups white sugar, 2 tablespoon brown sugar, packed, ½ teaspoon vanilla extract
With the paddle attachment, begin mixing the dough on the stir speed. Mix for about 3 minutes until the dough is crumbly and in small pea sized bits.
Preaheat the oven to 350. Line a 4x8 loaf pan with parchment so that some of the parchment hangs over the long sides of the pan.
Press ¾ of the shortbread crumble (about 2 ½ cups) into the bottom of the loaf pan. Then smear the cooled jam on top. Finally, sprinkle the remaining crumbles of shortbread on top of the jam, leaving some of the jam visible.
Bake for 35-40 minutes, or until the corners and sides of the shortbread start to get golden brown. Let cool completely before removing from the pan. Place the bars in the freezer for 15-20 minutes to harden before cutting for clean cuts.