Preheat oven to 350°F. Line two baking sheets with parchment paper or coat with nonstick cooking spray.
Whisk together sugar, prune puree, yogurt, oil and egg whites in a medium bowl. Stir together flour, cocoa, salt, baking soda and instant coffee in another medium bowl. Stir the dry ingredients into the wet with a wooden spoon just until blended. Drop the dough by heaping tablespoonfuls onto prepared baking sheets, spacing cookies about 1 1/2 inches apart. Bake for 12 to 14 minutes, or until the tops spring back when touched lightly. Transfer the cookies to wire racks and cool.