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1
Position rack in center of oven; preheat to 350F.
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2
Coat a 10-inch (12-cup) Bundt pan, preferably nonstick, with cooking spray.
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3
Spread walnuts in a small baking pan and toast in the oven until fragrant, 4 to 6 minutes.
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4
Let cool.
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5
Place walnuts, whole-wheat flour, all-purpose flour, baking powder, baking soda and salt in a food processor; process until walnuts are finely ground.
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6
Combine buttermilk, oil, rosemary, vanilla and lemon zest in a glass measuring cup.
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7
Place eggs, egg whites and sugar in a large mixing bowl.
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8
Beat with an electric mixer on high speed for 5 minutes.
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9
The mixture should be thickened and pale.
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10
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.
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11
Scrape the batter into the prepared pan, spreading evenly.
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12
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.
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13
Let cool in the pan on a wire rack for 5 minutes.
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14
Loosen the edges and turn the cake out onto the rack to cool completely.
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15
Meanwhile, make Vanilla Cream.
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16
To serve, set the cake on a platter and dust with confectioners' sugar.
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17
Garnish with berries and rosemary sprigs.
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18
Serve each piece with a dollop of Vanilla Cream and some berries.
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19
Vanilla Cream:
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20
Line a sieve or colander with cheesecloth and set over a bowl, leaving at least 1/2-inch clearance from the bottom.
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21
(Alternatively, use a coffee filter lined with filter paper.)
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22
Spoon in yogurt, cover and let drain in the refrigerator for 1 hour.
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23
Discard whey.
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24
Whip cream in a small bowl until soft peaks form.
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25
Add sugar, if using, and continue whipping until firm peaks form.
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26
Fold in the drained yogurt.
