Kasutera is a traditional Japanese sponge cake, usually flavored with honey. There are now many kinds of kasutera, made with various ingredients. For example, there are kasuteras made with powdered green tea, cocoa, and also brown sugar.
Preheat the oven to 170C/340F. Line two 28x10cm loaf pans with parchment paper. Sift the flour and salt and set aside.
Place the egg yolks, honey, 200 grams of sugar and vanilla in a bowl. Set the bowl over a pot of simmering water and beat at high speed until the mixture is thick and pale. Fold in the sifted flour in two additions.
Beat egg whites electric mixer on a low speed one minute, and then add in lemon juice. Increase speed to medium-high and continue to beat until foamy. Sprinkle in 20 grams of sugar and beat until stiff but not dry. With a rubber spatula, fold the egg whites in thirds. Pour batter into two prepared pans. Bake the cake in the center of hot oven for 35-45 minutes.