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.

In a mixing bowl, combine melted unsalted butter with granulated sugar until well blended. Add vanilla extract, salt, and lemon zest. Gradually incorporate all-purpose flour until a soft dough forms. Press this dough evenly into the bottom of the prepared baking pan.

In another bowl, beat softened cream cheese until smooth. Add one cup of granulated sugar and more lemon zest. Blend in eggs one at a time, then add yogurt or sour cream and fresh lemon juice. Sprinkle in salt and mix in additional flour until just combined.

Gently fold in blueberries into the cream cheese mixture. Pour this filling over the pre-baked shortbread crust.

Bake in the preheated oven for about an hour until set and golden brown on top. Allow to cool completely before chilling in the refrigerator.

Once chilled, lift out using the foil overhang and slice into squares.