Preheat oven to 300 degrees F. Beat egg whites with salt and cream of tartar until foamy. Add 1 cup of sugar slowly, and beat until meringue stands in very stiff peaks (about 10 to 15 minutes). Add vanilla and mix thoroughly. Fold in potato chips. Cover cookie sheet with unglazed paper (baking parchment or brown paper bag). Spread meringue on paper in 2 rounds 9 inches in diameter and about 3/4-inch thick. Bake 45 minutes. Cool. Place chocolate and milk in saucepan. Cook and stir over medium heat until chocolate is melted. Beat with electric mixer until blended. Combine 1/3 cup sugar, flour, and salt. Add to chocolate mixture slowly, stirring well. Cook and stir until thick, about 10 to 15 minutes. Add slightly beaten egg yolks slowly to chocolate mixture, blending well. Cook for 3 minutes longer, stirring constantly. Remove from heat. Stir in butter and vanilla. Cool to room temperature. Spread cooled chocolate filling on one meringue, then place second meringue on top. Spread sides with sweetened whipped cream. Chill until ready to serve, but don't keep in refrigerator mor than a few hours, lest the meringues wilt. Serve with more whipped cream or vanilla ice cream.