Melt butter in small saucepan over low heat. Pour into small mixing bowl; set nearby. Mix eggs, sugar, vanilla and salt in a large mixing bowl. Using an electric mixer, whip egg mixture until it has tripled in volume, about 4 to 5 minutes. Fold flour into mixture, one third at a time, just until incorporated. Pour about 1 cup of batter into the melted butter, and fold just until combined. Return butter mixture to reserved batter, and again fold to combine.

Pour batter into prepared pan. Smooth batter evenly. Bake 20 to 22 minutes or until top springs back slightly when lightly touched. Cool 10 minutes, then run a table knife blade around the outside edge of cake, freeing the sides and allowing air to get under the layer. Invert cake onto rack and allow to cool completely.