Step 1: Prepare the Cranberry Layer
Rinse and sort the cranberries, discarding any that appear damaged or squishy. In a medium saucepan, combine the cranberries, water, and 6 tablespoons of granulated sugar. Bring the mixture to a boil over medium-high heat, then reduce the heat to medium-low. Cover the saucepan with a splatter screen to prevent splattering. Stir occasionally at first, then constantly to prevent burning. Simmer for 10-15 minutes, or until all the cranberries have broken down. Set aside to cool for at least 30 minutes.
Step 2: Prepare the Baking Dish and Preheat the Oven
Preheat the oven to 325°F (160°C). Line a 9-inch square baking dish with parchment paper, ensuring there are no gaps or spaces. Allow a little overhang of parchment paper to make it easier to remove the bars later. Grease the parchment paper and the dish with butter or non-stick cooking spray. Set aside.
Step 3: Prepare the Shortbread Crust
In a medium bowl, combine the melted butter, 1/4 cup of granulated sugar, vanilla extract, and salt. Mix until well combined. Gradually mix in the flour until a thick dough forms. Press the dough firmly into the prepared pan in an even layer.
Step 4: Bake the Shortbread Crust
Bake the crust for 16-18 minutes, or until it is lightly browned. Remove the crust from the oven and poke it all over with a fork to prevent it from puffing up. Turn off the oven. Let the crust cool for about 20 minutes.
Step 5: Prepare the Lemon Layer
While the crust is cooling, in a medium bowl, combine the remaining 1 cup of granulated sugar and 3 tablespoons of flour. Whisk in the eggs until well combined. Add the fresh lemon juice and mix until the mixture is smooth. Set aside.