Preheat oven to 350 degrees. Prepare an 8x8-inch pan by lining with parchment paper. Extend the parchment paper up both long sides of the pan so that you can easily lift the finished bars out in one piece to cut. I also leave extra on the short sides, about half way up, just enough to hold the crust and filling.
Mix flour, salt, and powdered sugar in a medium bowl. Drizzle in melted butter and mix until it becomes a dough.
Pat into the bottom of the prepared pan. Bake 20 minutes then set on a wire rack to cool slightly.
While the shortbread crust is baking, beat or whisk eggs then add all remaining ingredients for the filling and mix until smooth.
Pour into slightly cooled cookie crust and return pan to oven for an additional 20-25 minutes of baking time. The filling should slightly jiggle when you move the pan.
Remove from oven to wire rack and let cool for 1 hour then set in refrigerator for 1-2 hours to make it easier to cut.
When ready to cut, lift out the entire bar by the parchment paper to a wooden cutting board. Dust with powdered sugar. Cut into 16 pieces. Store in the refrigerator in a covered container, with wax paper between layers, or place back in the baking dish and cover with plastic wrap.
Notes
STORE/FREEZESTORE: in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 5 days.FREEZE: on a parchment paper-lined baking sheet with room between them. Once frozen, wrap them with a layer of plastic wrap then either a layer of aluminum foil or place the plastic-wrapped bars in a freezer-safe container or plastic freezer bag. They will last up to 3 months.