For the Shortbread:
1 cup (4 1⁄2 oz) all-purpose flour
1⁄4 cup confectioner s sugar
1 TBS lightly packed finely grated lemon zest
1⁄2 c (4 oz) chilled unsalted butter, cut into small pieces
For the Lemon Curd: (this can be used separately as they do in England for spreading on cakes or cookies or toast or mixed with yogurt)
1 c fresh lemon juice (squeezed from 4 to 6 lemons)
4 TBS (2 oz) unsalted butter, cut into 2 pieces
2 TBS heavy cream (I use regular milk and it works fine too)
1 c granulated sugar
4 large eggs
2 large egg yolks
1⁄4 tsp salt
1⁄4 tsp vanilla extract
wooden spoon to stir
Position a rack in the middle of the oven and heat to 350. Butter an 8 inch square baking pan.
In a medium bowl, mix dry ingredients for shortbread. Cut the butter into the mixture with a pastry blender or two table knives until the mixture resembles small peas. Knead the dough in the bowl just until it begins to come together. Transfer the dough to the baking pan and, with floured hands, press it evenly over the bottom. Bake until very light golden brown, about 20 minutes. Let cool on a rack while you make the filling.
In a medium saucepan, heat the lemon juice, butter and cream to just under a boil; the butter should be melted. Remove from the heat.
In a medium bowl, whisk together by hand the sugar, eggs, yolks until combined. Whisk in a bit of the hot liquid and then gradually whisk in a bit more until it s all added. This technique is called tempering and heats the eggs slowly and gently so they don t curdle.
Pour the mixture back into the saucepan and heat on medium, stirring constantly with a wooden spoon, scraping the bottom and sides of the pan to keep the eggs from scrambling. Cook until the lemon curd coats the spoon thickly enough to leave a line when you draw your finger through on the back of the spoon, about 5 to 8 minutes (may take longer).
Remove from heat and strain through a fine sieve if any egg has curdled. Stir in the salt and vanilla.
To finish: Pour the curd over the baked shortbread and smooth it evenly with a spatula, if needed. Bake until the curd has set and jiggles like firm jello, 15 to 20 min. Let cool to room temperature. Refrigerate until completely set, at least 4 hours. Cut into 16 pieces.