In a small saucepan, mix cornstarch, 3/4 cup sugar, milk, and egg yolks. Cook, stirring, until bubbly and thick. Remove from heat, and add vanilla. Divide mixture in half. Add chocolate chips to one half, and stir until melted and smooth. Pour into pastry shell. Chill.
Stir gelatin mixture into the other half of the hot egg yolk mixture. Stir in rum. Chill until slightly thick.
Beat egg whites until soft peaks form. Gradually add remaining 1/2 cup sugar, and beat to stiff peaks. Fold into partially set gelatin/rum mixture. Chill until mix will mound, then spoon into pie shell on top of chocolate layer. Chill overnight.