The Ultimate Lemon Butter Bar
From Rose’s Christmas Cookies by Rose Levy Beranbaum (HarperCollins, 1998)
Adapted by StarChefs

Yield: 18 Servings

Ingredients:

Shortbread base:
• 10 Tablespoons unsalted butter
• 2 Tablespoons powdered sugar
• 2 Tablespoons granulated sugar
• 1 ¼ cups bleached all-purpose flour

Lemon curd topping:

• 2 teaspoons lemon zest, finely grated
• 4 large egg yolks
• ¾ cup sugar
• 3 ounces lemon juice, freshly squeezed
• 4 Tablespoons unsalted butter, cut into pieces
• Pinch of salt
• 2 Tablespoons powdered sugar, for dusting

Method:
Line the bottom and two sides of an 8-inch by 8-inch by 2-inch metal baking pan with a strip of aluminum foil. Place 1 oven rack in the middle of the oven. Preheat oven to 325°F for at least 20 minutes before baking. If using a glass pan, lower the oven temperature by 25°F.

For shortbread base:
Cut butter into 1-inch cubes and refrigerate. In a food processor with metal blades, process the sugar 1 minute or until very fine. Add butter and pulse until the sugar disappears. Add flour and pulse until moist, crumbly pieces form, and no dry flour particles remain. Transfer mixture into a plastic bag and press it together. Remove from bag and knead dough lightly until it holds together.

Pat the dough into the prepared pan. Use a fork to prick the dough all over.
Bake for about 30 to 40 minutes or until the edges are lightly browned and the top is pale golden (do not brown). While the shortbread is baking prepare the topping.

For lemon curd topping:
Have ready near the range a strainer, suspended over a medium bowl containing the lemon zest.

In a heavy non-reactive saucepan, beat the yolks and sugar until well blended. Stir in the lemon juice, butter and salt. Cook over medium-low heat, stirring constantly (be sure to scrape the sides of the pan), until thickened and resembling hollandaise sauce, which thickly coats a wooden spoon but is still liquid enough to pour. The mixture will change from translucent to opaque and begin to take on a yellow color on the back of a wooden spoon. It must not be allowed to come to a boil or it will curdle. Whenever steam appears, remove the pan briefly from the heat, stirring constantly, to keep the mixture from boiling. When the curd has thickened, pour it at once into the strainer. Press with the back of a spoon until only the coarse residue remains. Discard the residue (or enjoy it as a treat--it tastes great). Stir gently to mix in the zest sitting in the bowl.

When the shortbread is baked, remove it from the oven, lower the temperature to 300°F, pour the lemon curd on top of the shortbread, and return it to the oven for 10 minutes. Cool the lemon curd-topped shortbread completely in the pan on a wire rack. Refrigerate the pan for 1 hour to set the lemon curd completely before cutting into bars.

Run a small metal spatula between the sides of the pan and the pastry on the 2 sides without the aluminum foil. Use the foil to lift out the lemon curd-covered shortbread onto a cutting surface. Using a long sharp knife, cut the shortbread first in thirds, then in half the other way, and then cut each half in thirds. Wipe the blade after each cut.

Place powdered sugar in a strainer and tap the strainer with a spoon to sprinkle a thick, even coating over the lemon curd. (The powdered sugar will start to be absorbed into the lemon curd after several hours, but it can be reapplied before serving. The lemon bars may be stored in an airtight container for 3 days at room temperature, 3 weeks in the refrigerator, or 3 months in the freezer.

Smart Cookie Tips:
Cooking the topping before pouring it onto the shortbread ensures crispness as opposed to the usual pastiness of the pastry.

Returning the curd to the oven, where it will be exposed to heat without stirring, causes the yolk to rebond, making it firm enough to cut after cooling.

If each lemon is heated about 10 seconds in a microwave oven on high power and rolled around while pressing on it lightly, it will release a significantly greater quantity of juice.

Sugar raises the coagulation point of the egg yolk. It also protects it from premature coagulation during the addition of the lemon juice. If the juice were added directly to the unprotected yolk, the yolk would partially coagulate and, when strained, a large percentage of it would be left behind in the strainer. Be sure to mix the sugar well with the egg yolks before adding the juice.