Preheat oven to 350º F. Generously butter a 9-inch square metal baking pan. Add a layer of parchment paper, leaving the edges long enough to lift the bars from the pan once cool for easier slicing. Butter the parchment paper, pressing it down to help it adhere smoothly to the pan. Set aside.
To make the shortbread crust, combine flour, confectioners' sugar, salt, rosemary, and pine nuts in the bowl of a food processor and process until the pine nuts are well chopped. Add butter and process in spurts until the mixture is pale yellow and resembles course meal. Sprinkle the loose mixture into the prepared pan and press down on it with your fingers or the bottom of a measuring cup to create a firm, even layer over the bottom of the pan.
Bake on the middle rack until pale golden brown, about 20 minutes.
Begin making filling when the shortbread goes in the oven. Whisk together egg yolks and whole eggs in a medium saucepan. Add granulated sugar and whisk briefly to combine. Add lemon juice and salt and whisk until combined. Add the butter slices and cook over medium-low heat, stirring constantly with a wooden spoon. Continue cooking until the filling thickens to a thin sauce-like consistency and registers 170º F on an instant-read thermometer, about 5 minutes. Immediately pour the curd through a mesh stainless steel strainer set over a nonreactive bowl. Stir in heavy cream and lemon zest and then pour the filling onto the warm crust.
Bake until the filling is shiny and opaque and the center jiggles slightly when shaken, 10 to 15 minutes. Cool to room temperature and then refrigerate for at least an hour.
Remove bars from the pan using the parchment handles. Slice into 2 1/4-inch squares, wiping knife between cuts if necessary. If desired, sprinkle powdered sugar over the bars using a mesh strainer before serving.