Combine the cold butter cubes, the flour, sugars, lemon zest, and salt into the bowl of a food processor. Pulse until a crumbly dough forms.
If the dough is a bit too crumbly to be pressed together, add ice water 1 teaspoon at a time until the correct consistency is achieved. Dough should have a wet mealy texture.
Prepare the pan for baking. Lay a rectangle of parchment paper inside a 9x9 pan, so the paper hangs from two edges. This will help in lifting the bars out of the pan without damaging them.
Preheat the oven to 325F and press the dough into the prepared pan to create a uniform, even surface.
Bake for 30 minutes.
Measure out all ingredients prior to cooking, and be sure they are easily accessible. Prepare a strainer above a large bowl.
Whisk together the lemon juice, eggs, sugar, and cornstarch in a pot over medium heat. Stir frequently until the liquid reaches boiling.
The second you see bubbles from boiling, remove the mixture from the heat to avoid curdling the eggs. Strain the mixture into a large bowl and whisk in the lemon zest and olive oil.
When the crust is done baking, pour the curd over it and spread until it forms an even layer.
Bake for 10 minutes.
Refrigerate until the bars are completely cooled. [This could be several hours.]
Once cool, remove from the pan using the parchment paper and cut into small squares. [A 5 x 4 grid will allow for 20 small servings.]
Sift powdered sugar on top and garnish with rosemary.