• Position a rack in the center position of the oven. Preheat the oven to 350ºF (177ºC).
• Line an 8″ x 8″ (or 9" x 9") baking pan with foil. Leave an overhang on the sides. Set aside.

SHORTBREAD CRUST

• Stir the melted butter, granulated sugar, vanilla extract, salt, and the lemon zest together in a medium bowl. Add the flour and stir until combined.
• Reserve ¾ cup² of the shortbread crust and place in the refrigerator or freezer until needed.
• Press remaining crust evenly into the prepared pan. Bake for 18 minutes while you prepare the filling.

FILLING

• In the bowl of a stand mixer fitted with the paddle attachment, or in a medium sized bowl with a hand mixer, beat cream cheese on high until smooth. Add the sugar and lemon zest and beat until smooth. Add the eggs, and continue beating until completely incorporated. Add the yogurt, lemon juice, and salt and beat again. Finally, add the flour and beat one more time until just combined. Scrape down the sides as necessary before adding the blueberries, then gently fold in the blueberries with a large spatula.
• Pour filling into prepared crust and add a few blueberries on top of filling. Remove reserved crust from refrigerator and crumble over top of the filling (you may need to separate into crumbs with a knife or fork). Sprinkle with additional sugar or course sugar, if desired. 
• Bake bars for 55-60 minutes minutes, or until a toothpick inserted in center comes out clean. Allow bars to cool at room temperature on a wire rack. When completely cool, chill in the refrigerator for 1 hour.
• When bars are completely chilled, lift out of the pan using foil overhang and cut into squares. Top with lemon zest before serving, if desired. Bars can be stored covered in the refrigerator up to 5 days. Bars freeze well, up to 3 months. Thaw in the refrigerator overnight.