Place the thawed berries into a small bowl along with 1/8 cup of the sugar. Using the back of a large spoon, mash the berries against the side of the bowl until you have a purée and stir to mix well with the sugar. Set aside.
In the bowl of a stand mixer, pour in the heavy cream and remaining ¼ cup of sugar. Whip on high until stiff peaks form.
Fold the raspberry purée into the whipped cream in spoonfuls, stirring just until you see ribbons of raspberry throughout the whipped cream. (If you want, you can hold back stirring in some of the raspberry purée to get the ribbon effect you desire, and use any leftovers as a topping.)
Spoon into individual serving glasses and chill 1 hour before serving.