Line a 9 x 13 metal baking pan with parchment paper or spray with cooking spray and preheat the oven to 350° F (176° C). If a glass dish is being used instead, preheat the oven to 325° F (162° C).
In a large bowl, add the crust ingredients and with a pastry cutter, blend until the butter is pea-sized. A food processor can be used as well; be judicious so as to not over-process.
Dump the crust ingredients into the baking pan, spread it evenly, and press it in firmly. Par-bake the crust for 20 minutes.
Zest and juice the lemons then finely chop the zest.
About halfway through the crust baking, whisk the lemon topping ingredients, the sugar, eggs, lemon juice and zest, thoroughly in a medium bowl and set aside for 10 minutes.
Remove the baking pan from the oven, thoroughly whisk the lemon topping ingredients to mix completely, then pour over the shortbread crust. Carefully move the pan back into the oven and bake for 20-25 minutes, until the topping is set and browning along the edges is seen.
Cool the bars in the pan completely. Cut to serve and sprinkle with powdered sugar.