3/4 cup (1 1/2 sticks) unsalted butter, at room temperature
3/4 cup sugar
1/2 teaspoon salt
1 tablespoon grated lemon zest
2 tablespoons fresh lemon juice
1 large egg
1 tablespoon vanilla extract
1 3/4 cups all-purpose flour
1/4 cup poppy seeds
In a large mixing bowl with a handheld mixer or the bowl of a stand mixer fitted with a paddle attachment, combine the butter, sugar, salt, and zests. Cream on medium speed until pale and fluffy, 3 - 4 minutes. Add the egg and vanilla and mix to combine. Scrape the bottom and sides and continue mixing to combine all of the ingredients. Turn the mixer off, add the flour all at once, and then turn the mixer on its lowest speed. Mix just until all of the ingredients are combined and form a smooth dough. Turn the dough out onto a sheet of parchment paper and roll into a log about 1 1/2-inches in diameter. Spread the poppy seeds out on the parchment and roll the log in them to evenly coat the dough on all sides. Roll the dough up in the parchment and chill until firm, about 2 hours. (NOTE: the dough can be made up to this point and kept, tightly wrapped, in the refrigerator for up to 10 days.) Pre-heat the oven to 375ºF. Line two baking sheets with parchment paper and set aside.
Slice the chilled log of cookie dough into cookies about 1/4-inch thick. Arrange the cookies on a baking sheet leaving about 2-inches between each cookie. Bake until the bottoms are pale golden brown, 13 to 15 minutes, rotating the sheets halfway through baking. Cool completely before serving. The cookies can be stored in an airtight container at room temperature for up to 5 days.