• Position a rack in lower third of oven and preheat oven to 350 degrees. Line bottom and sides of 8- by 8-inch baking pan with aluminum foil.

To make crust:

• In a medium bowl, combine melted butter with sugar, vanilla and salt. Add flour and mix until just incorporated. Press dough evenly over bottom of pan. Freeze dough for 15-20 minutes until firm. Start filling while dough chills.
• Bake 25 to 30 minutes, or until crust is fully baked, well-browned at the edges and golden brown in the center. If dough bubbles poke a few holes in it andpat it down.

To make topping:

• While crust is baking, stir together sugar and flour in a large bowl until well-mixed. Whisk in eggs. Stir in lime or lemon zest and juice. When crust is ready, reduce heat to 300 degrees, slide rack with pan out and pour filling onto hot crust. Bake 20 to 25 minutes longer, or until topping no longer jiggles when pan is tapped.
• Remove from oven to a wire rack to cool completely. Lift up foil liner and transfer bars to a cutting board. If surface is covered with a thin layer of moist foam (not unusual), blot surface gently with a paper towel to absorb excess moisture; repeat with a fresh piece of paper towel if necessary.
• Using a long, sharp knife, cut bars into 16 or 24 daintier bars and sift powdered sugar over bars, if desired. Stored in an airtight container, bars can be refrigerated for up to 3 days.
• I bake my lemon bars in a rectangular tart pan with removable bottom. It gives the bars a nice side crust and a nice finished look. The idea came from the blog Baking Obsession Check it out for instructions to bake in a rectangular pan.