To a small saucepan, add the raspberries, sugar and 3 tbsp. water.
Bring to a slow simmer, over low heat. Simmer gently, for about 10 minutes, using a rubber spatula to stir.
A very gentle hand is needed here, both with the simmering and the stirring, so that the raspberries do not break down.
Dissolve the corn starch in only enough water to get it to a liquid state.
Pour into the simmering raspberries, stirring gently until the mixture is thickened; about another minute.
Remove from the heat and allow the sauce to cool to room temperature.