Foolishly simple to make, this old-fashioned English dessert marries a fruit purée with whipped cream. To cut down on calories, we folded drained vanilla yogurt into a small amount of real whipped cream.
Combine rhubarb, 1 cup berries, sugar, orange juice and cinnamon in saucepan. Bring to a simmer over medium-high heat, stirring constantly. Reduce heat to medium low, cover, and cook 5 minutes, stirring often. Remove lid and cook 3 to 5 minutes, or until rhubarb is tender and mixture has broken down into a chunky purée, stirring often. Cool completely.
Meanwhile, line sieve or colander with cheesecloth or coffee filters, and set over bowl. Spread yogurt in sieve, and let drain in refrigerator 1 to 1 1/2 hours, or overnight.
Chill mixing bowl and beaters in freezer 1 hour, or overnight. (This will make it easier to whip cream.) Put remaining 1/2 cup blackberries in refrigerator.
Fold yogurt into rhubarb mixture with spatula. Whip cream in chilled bowl with electric mixer until soft peaks form. Fold into rhubarb-yogurt mixture, leaving distinct swirls. Spoon into dessert dishes, and garnish with wafer crumbs and thawed blackberries.