Add the vanilla, egg yolk, and salt. Mix until blended.
Mix in flour at low speed until incorporated.
Halve the dough and pat into two squares. Wrap in plastic wrap; refrigerate for 30 minutes.
Preheat oven to 350°F Line 2 cookie sheets with parchment. Remove the parchment from on cookie sheet and dust with flour. Roll one dough square into 17"x11" rectangle about 1/8" thick. Transfer the parchment and pan to the refrigerator while you repeat with dough.
Using a fluted pastry wheel, trim all sides of the chilled dough to even it out. Cut the dough into 15 squares. Separate the squares on the pan. Repeat with other pan.
Bake until lightly brown on the edges, 11 to 12 minutes. Transfer to racks to cool.
Sandwich the cookies with the lemon curd; dust with confectioner's sugar.