Put raspberries in a small saucepan and bring to a low boil. Add sugar and zest and stir to dissolve sugar. Taste, if too tart, add a little more sugar.
The mixture should now have lots of juice. If not, add some water and bring to a low boil.
Dissolve cornstarch in a teaspoon of water and add to raspberry mix. Stir until mixture thickens slightly. It should be thick enough to coat the back of a spoon.
Remove from heat and cool to room temp.
In a separate bowl, mix equal parts of yogurt and Cool Whip. Mix well.
Spoon the yogurt mixture into 4 tall wine glasses.
Top each with 1/4 of the raspberry mixture and chill in frig until time to serve.
A sprig of mint or a lemon peel curl is a nice garnish.