Preheat the oven to 350°F (convection).

In a medium saucepan, combine the blueberries and sugar. Add the lemon juice, if using.

Place the saucepan over medium-low heat and stir until the mixture begins to bubble.

Once bubbling, stir in the cornstarch–water mixture while continuing to cook.

Let the jam simmer on medium-low heat for 3–4 minutes, stirring every minute or so, until thickened. Remove from heat and set aside.

In a separate bowl, whisk the butter until light and off-white.

Add the sugars and mix until fluffy.

Mix in the vanilla extract, lemon extract, and lemon juice.

Add the salt and flour and mix just until a soft dough forms.

Butter and line a 9×9-inch pan with parchment paper (binder clips can be used to secure the sides, if oven-safe).

Press about ¾ of the dough evenly into the bottom of the pan.

Spread the blueberry jam over the crust, then crumble the remaining dough evenly on top.

Bake for 42–44 minutes, until lightly golden and set.

Allow the bars to cool completely before dusting with powdered sugar, if desired. Slice and serve.

Store in the refrigerator for up to 10 days, or at room temperature for up to 8 days.