Step 1To make the yogurt cheese, place the yogurt in a fine-mesh strainer lined with cheesecloth. Fold the cheesecloth to cover the yogurt and let it stand over a deep bowl in the refrigerator for 6 to 8 hours or overnight. Discard any liquid in the bowl.
Step 2Heat the oven to 350 degrees.
Step 3Lightly butter the bottom of a 10-inch springform pan. Sprinkle the vanilla wafer crumbs evenly over the bottom of the pan and pat them firmly in place.
Step 4Combine 2 cups of the yogurt cheese, the ricotta cheese, 3/4 cup of sugar, nonfat milk, egg whites, cake flour and vanilla in the bowl of a food processor. Process the cheese filling until blended. Stir in the ginger and pour into the crust.
Step 5Bake the tart just until set, 40 to 45 minutes. Cool to warm.
Step 6Combine the raspberries, strawberries and the remaining 2 tablespoons of sugar in a food processor or blender and puree. Serve with the tart.