Mix together flour and salt, set aside. Beat egg whites until frothy, then add vinegar. Continue beating until egg whites are stiff or just until spoon can be turned over without whites slipping out. Add sugar gradually beating after each little bit.

Beat egg yokes until thick, lemon colored. Add vanilla and beat again. Fold yokes into whites with a spatula, using long deep strokes and as few as possible. Sift in flour, folding it in gradually.

Pour into buttered and floured 8x8" pans. Spread batter 1/2" thick. Bake in hot oven at 425 degrees for 8 minutes or until cake springs back when touched lightly with a finger.

Let cool for a few minutes, then flip pan over a flat surface and cake should come out. Spread with your favorite jam, then carefully roll up. Slice 1" rolls.