I like bold bars, racing with extra stuff. These have a layer of raspberry preserves bridging the shortbread base with its tangy topping.
Citric acid is available in drugstores and kosher food sections.
You may omit it, but the tang will be much reduced. You may also use a 9 by 9 inch square pan. For the crust, cream or blend the butter, flour, confectioner's sugar, baking powder, lemon zest and salt until a soft dough forms. Press evenly into prepared pan. Chill 10 minutes.