Preheat the oven to 325F (170C).
Line an 8x8 inch (20x20 cm) pan with parchment paper, leaving an overhang around the edges.
Ensure your cherries are pitted (I use a cherry pitter for this) and slice each cherry in half or quarters.
In a medium bowl, toss together the ingredients for the cherry filling: cherries, 1 tablespoon sugar, 2 tablespoons cornstarch and fresh lemon juice. Set aside.
In a separate large bowl, whisk together the dry ingredients for the shortbread layer: 1 ¾ cups all-purpose flour, ⅔ cup granulated sugar, 3 tablespoons cornstarch and ¼ teaspoon salt (leave out the 1 tablespoon brown sugar - you'll use this later).
Stir/whisk the melted butter into the flour mixture. I use a fork for this. It should be crumbly.
Press a little more than half of the shortbread layer into the prepared pan.
Bake in the preheated oven for 15-20 minutes, or until the top looks set. Remove from the oven, but leave the oven turned on.
With the reserved shortbread mixture (that's still in the bowl), stir in the 1 tablespoon of brown sugar.
Carefully spoon the cherry mixture on top of the baked shortbread base to create an even layer. Be careful, as the pan will be hot. If there is a lot of juice at the bottom of the bowl - do not add it to the pan.
Then crumble the remaining shortbread/crumb mixture over top of the cherry layer. If you can see some of the cherries poking through, that is ok.
Place the pan back in the preheated oven and bake for about 25 minutes or until the top looks baked and is started to turn golden yellow. You may see some of the cherry juice bubbling through.
Cool the bars completely (this will take 3-4 hours).
When ready to slice, lift the bars out of the pan using the overhang of the parchment paper and set on a cutting board. Slice using a sharp knife, being sure to cut all the way through the shortbread base.