Place water, cranberries, and 6 tablespoons of sugar in a medium saucepan, bring to a boil over medium-high heat. Reduce heat to low, simmering, stirring occasionally, until cranberries start to burst. About 15 minutes.

Continue to simmer, stirring and breaking up any whole cranberries - a wooden spoon works great for this. Cook until cranberries are completely broken down, with no whole pieces remaining, about another 10 minutes. Remove from heat, cooling for 30 minutes.

Meanwhile, preheat your oven to 350°F (175°C). Lightly spray a 9-inch square baking dish with spray oil. Line the pan with two sheets of parchment paper—one going across the pan and the other in the opposite direction—so the paper hangs over all four sides. Lightly spray between the sheets to help keep them in place.

In a medium-large mixing bowl, stir together 1 ⅓ cups flour, ¼ teaspoon salt, powdered sugar, ½ cup granulated sugar, and lemon zest until combined. Mix in butter cubes, using a pastry cutter or your fingers (you may also do this step in a food processor). Once the mixture looks like sandy crumbs that stick together when pinched, press into the bottom of the prepared pan. Bake for 20-30 minutes until golden brown.

Using a fork, poke holes into the warm shortbread crust this will help the layers stick. Allow to cool slightly, about 20 minutes (do not turn off oven).

Once cooled slightly, spread the cooled cranberry mixture over the crust, chill for 30 minutes, meanwhile make the lemon layer.

In a medium-large mixing bowl, whisk together the lemon juice, 1 ¼ cups granulated sugar, ⅓ cup flour and ¼ teaspoon salt. Add lightly beaten eggs to the mixture, whisking until combined, frothy and lighter in color. Set aside until ready to use.

Once the cranberry layer has chilled on the crust, remove from the fridge and give the lemon layer one last good whisk. Pour evenly over the top of the cranberry mixture. Bake in a 350°F (175° C) oven 40-45 minutes until the center is set, it may jiggle slightly.Remove to wire rack, cool for 1 hour, then refrigereateuntil lemon layer is set, at least 4 hours or overnight.

Using the parchment paper to lift the bars from the pan, place on a cutting board and cut into 9, 12 or 16 squares.

For clean slicing, run a sharp knife under hot water, wipe dry and slice, repeating between slices. Sprinkle if desired with powdered sugar before serving.