Cake: Preheat oven to 350 degrees. Butter a 9-by-13-inch baking pan. Beat butter with sugar, baking powder, baking soda, salt, and lemon zest on medium-high speed until light and fluffy, about 3 minutes. Add eggs, one at a time, beating to combine after each addition and scraping down sides as needed. Beat in lemon juice. Beat in flour in three additions, alternating with milk and beginning and ending with flour. Scrape batter into prepared pan, smoothing top with an offset spatula.

Bake until a tester inserted in center comes out with a few moist crumbs, 30 to 35 minutes. Transfer pan to a wire rack; let cool completely.

Glaze: Combine sugar, cornstarch, salt, and lemon zest in a saucepan. Whisk in yolks, then 1 2/3 cups water, lemon juice, and butter. Bring to a boil over mediumhigh heat, whisking constantly, and cook, still whisking, 1 minute. Strain through a fine-mesh sieve into a heatproof bowl. Let stand 30 minutes, whisking occasionally.

Poke 20 holes in cake with a skewer; pour glaze over top. Refrigerate at least 2 hours and up to overnight. Decorate with candied zest. (Leftovers can be refrigerated, wrapped in plastic, up to 3 days.)