Ingredients
1 1/2 cups all-purpose flour
2/3 cup sugar
1 teaspoon baking powder
1/2 teaspoon coarse salt
2 tablespoons poppy seeds
1 stick unsalted butter, melted and cooled
1 large egg yolk
1/2 teaspoon pure vanilla extract
1 stick plus 2 tablespoons unsalted butter, softened
3/4 cup sugar
1 large egg
6 large egg yolks, 3 egg whites reserved for Swiss Meringue, below
3/4 cup fresh Meyer lemon juice or regular lemon juice (from 4 to 5 lemons)
3 large egg whites (reserved from Meyer Lemon Curd, above)
3/4 cup sugar
Pinch of cream of tartar
Pinch of coarse salt
Piping bag fitted with a 1/2-inch round tip, or 1-gallon freezer bag
Preparation
Cookies: Whisk together flour, sugar, baking powder, salt, and poppy seeds in a large bowl. Make a well in center of mixture. Add butter, egg yolk, and vanilla to well, stirring with a wooden spoon or rubber spatula to combine. Knead a few times to form a cohesive dough, then transfer to a sheet of parchment or waxed paper. Form dough into a 12-inch-long, 1 1/4-inch-diameter log. (It’s okay if it cracks in places.) Wrap in parchment, pressing top and sides to ensure a tight, uniform log. Refrigerate until firm, at least 1 hour and up to 2 days.
Preheat oven to 350 degrees with racks in upper and lower third. Unwrap dough, cut into 1/4-inch-thick rounds, and space 1 inch apart on parchment-lined baking sheets. Bake, rotating pans halfway through, until cookies are firm in center and golden brown around edges, 12 to 15 minutes. Cool on baking sheets 5 minutes, then transfer to wire racks to cool completely.
Curd: Beat together butter and sugar with a mixer on medium-high speed until light and fluffy. Reduce speed to low and add whole egg. When thoroughly combined, gradually add egg yolks, then lemon juice. (Mixture will appear curdled.) Transfer to a medium saucepan and cook over medium heat, stirring constantly, until mixture has thickened and coats the back of a wooden spoon, 5 to 7 minutes. Refrigerate curd until cold, about 2 hours.
Swiss meringue: Whisk together egg whites, sugar, cream of tartar, and salt in a bowl set over a pot of simmering water until sugar is dissolved and a candy thermometer inserted into mixture reads 160 degrees. Remove from heat and beat with a mixer on high speed until stiff, glossy peaks form and mixture is cool, about 10 minutes.
Assembly: Place cookies side by side on a baking sheet. Top each with 1 teaspoon curd. Place meringue in a pastry bag fitted with a 1/2-inch round tip (Ateco #806) or a gallon freezer bag with 1 corner snipped to make a 1/2-inch hole. Pipe spirals of meringue on top of each cookie. Lightly brown meringue in places with a small kitchen blowtorch.
