Preheat oven to 325°. Whisk flour and powdered sugar in a medium bowl.
Beat egg whites, cream of tartar, and salt in the bowl of a stand mixer fitted with the whisk attachment on medium-low speed to break up egg whites, about 30 seconds. Increase speed to medium-high and beat until whites are very foamy and barely form soft peaks, about 45 seconds longer. Gradually add granulated sugar and continue to beat on medium-high speed until whites are firm and glossy and hold stiff peaks (if you overbeat, the meringue will look dry, grainy, and curdled). Beat in lemon juice and vanilla.
Remove bowl from mixer. Sift one-third of dry ingredients over meringue. Return bowl to mixer and beat on low speed until incorporated. Remove bowl from mixer again and repeat sifting and mixing process 2 more times. Remove bowl again and fold batter several times with a large flexible spatula, scraping and lifting batter from the bottom to make sure all ingredients are fully incorporated.
Scrape about half of batter into ungreased tube pan. Using a spoon or small offset spatula, push batter around bottom of pan, into corners, and up sides and center tube (this helps form smooth sides and eliminate air pockets); it’s okay if batter forms a little bit of a trough, with the center at a lower level than the outer edges. Scrape remaining batter into pan, spreading evenly and smoothing top.
Bake cake until top is puffed, split, golden, and springs back firmly when touched and a tester inserted into the center comes out clean, 35–40 minutes. Immediately invert pan, placing hollow part of tube over a long-necked wine or other glass bottle to elevate it (if your pan has feet, simply rest upside-down on feet). Let cool at least 1 hour, then turn right side up. The upside-down cooling is key for a cake that doesn’t deflate and sink onto itself. Using a thin metal spatula, cut around sides and loosen cake. Pull on tube to release cake from pan, then cut around tube. Using thin metal spatula, loosen cake from bottom. Invert cake onto a plate or cake stand and remove bottom. Serve with berries and whipped cream or lemon curd.