Dissolve instant coffee powder in quarter cup water; set aside. Sift together flour, half cup sugar, baking powder, and salt in a mixing bowl. Make a well in center; add oil, egg yolks, water and coffee mixture, and vanilla. Beat at high speed of an electric mixer about 5 minutes or until satiny smooth. Beat egg whites and cream of tartar in a large mixing bowl until soft peaks form. Add remaining half cup sugar, 2 tablespoons at a time, and beat until stiff peaks form. Pour egg yolk mixture in a thin, steady stream over entire surface of egg whites; then gently fold whites into yolk mixture. Pour batter into an ungreased 25 cm tube pan, spreading evenly with a spatula. Bake at 160ºC or 1 hour or until cake springs back when lightly touched. Invert pan; cool 40 minutes. Loosen cake from sides of pan using a narrow metal spatula; remove ----