Position rack in center of oven; preheat to 350 °F. Coat a 10-inch (12-cup) Bundt pan, preferably nonstick, with cooking spray.
Spread walnuts in a small baking pan and toast in the oven until fragrant, 4 to 6 minutes. Let cool.
Place walnuts, whole-wheat flour, all-purpose flour, baking powder, baking soda and salt in a food processor; process until walnuts are finely ground.
Combine buttermilk, oil, rosemary, vanilla and lemon zest in a glass measuring cup.
Place eggs, egg whites and sugar in a large mixing bowl. Beat with an electric mixer on high speed for 5 minutes. The mixture should be thickened and pale.
With a rubber spatula, alternately fold the dry ingredients and buttermilk mixture into the egg mixture, making 3 additions of the dry and 2 additions of liquid. Scrape the batter into the prepared pan, spreading evenly.
Bake the cake until the top springs back when touched lightly and a toothpick inserted in the center comes out clean, 35 to 40 minutes. Let cool in the pan on a wire rack for 5 minutes. Loosen the edges and turn the cake out onto the rack to cool completely.
Meanwhile, make Vanilla Cream.
To serve, set the cake on a platter and dust with confectioners' sugar. Garnish with berries and rosemary sprigs. Serve each piece with a dollop of Vanilla Cream and some berries.
Equipment: 12-cup bundt pan
Whole-wheat pastry flour is milled from soft wheat. It contains less gluten than regular whole-wheat flour and helps ensure a tender result in delicate baked goods while providing the nutritional benefits of whole grains. Store in the freezer. Available in large supermarkets and in natural-foods stores.
Dried egg whites are pasteurized so this product is a wise choice in dishes that call for an uncooked meringue. They are also convenient in recipes calling for egg whites because there is no waste. Look for brands like Just Whites in the baking or natural-foods section of most supermarkets.
To bring cold eggs to room temperature quickly, place in a bowl of warm water for a few minutes; they will beat to a greater volume.
To separate eggs safely: Use an egg separator, an inexpensive gadget found in cookware stores; separating eggs the traditional way, by passing the yolk back and forth between pieces of egg shell or your hands, can expose eggs to bacteria.