Step 1

Cake: Preheat oven to 350°F. Brush a 9-by-13-inch or 8-by-12-inch baking pan with butter. Line with parchment, leaving a 2-inch overhang on long sides. Butter parchment.

Step 2

Whisk together flour, baking powder, and salt. Combine eggs and sugar in the bowl of a mixer fitted with the whisk attachment. Beat on high speed until thick and pale, about 4 minutes. Add flour mixture; beat just to combine. Add butter, lemon zest, and vanilla; beat to combine. Add sour cream; beat until smooth.

Step 3

Scrape batter into prepared pan, smoothing top with a spatula. Bake until top springs back when lightly touched and a tester inserted in center comes out clean, 30 to 35 minutes (for 9-by-13-inch) or about 40 minutes (for 8-by-12).

Step 4

Let cool in pan on a wire rack 10 minutes. Using parchment overhangs, lift cake out of pan and let cool completely on rack. (Cake can be made to this point and stored at room temperature, wrapped in plastic, up to 1 day.)

Step 5

Meringue Frosting: Place 1 cup sugar, 5 tablespoons water, and cream of tartar in a small saucepan over medium heat. Bring to a boil, then cook 4 minutes. Meanwhile, beat egg whites on high speed to soft peaks, 2 to 3 minutes. Gradually add remaining 1/3 cup sugar, beating until sugar has dissolved. (If ready before sugar syrup, keep whites moving on lowest speed.)

Step 6

With mixer running, gradually pour hot syrup into meringue (aim for side of bowl, and avoid beater). Continue beating until cool to the touch and very fluffy, 4 to 5 minutes. Spread over top of cake. Lightly brown with a torch (or under broiler about 30 seconds). Frosted cake is best the day it's assembled.