Pour half of the buttermilk into a small saucepan. Add the cornstarch and stir well, using a wire whisk. Stir until the cornstarch is fully incorporated and there are no lumps. Cook over medium heat until thickened and bubbling (about 3-4 minutes). Remove from heat.
Pour buttermilk mixture into a medium bowl. Add remaining buttermilk, sweetener, mustard, reduced-fat mayonnaise and salt. Stir well. Add poppy seeds and stir well. Cover and refrigerate at least one hour or until ready to use.