1/2 cup cold water
4 tablespoons cornstarch
1/2 cup plus 2 tablespoons dark vinegar
2 tablespoons salt
1/2 cup granulated sugar
1 egg
4 tablespoons prepared mustard
4 tablespoons margarine, cut into tiny bits

Put all the ingredients into a blender and blend until very smooth.

Put blender contents in top of double boiler and cook over gently boiling water, stirring often for 12 to 15 minutes, until thickened and smooth.

Return mixture to blender and blend at high speed for 30 seconds or until smooth.

Put sauce into a covered container and refrigerate for 24 hours before using.

This will keep refrigerated for up to 3 months.