1) Preheat the oven to 350°F. Grease and flour three 9" round cake pans.
2) To make the cake: In a small bowl, whisk together the flours, baking powder, and salt.
3) In a large bowl, beat the butter until smooth.
4) Gradually add the sugar and beat until fluffy.
5) Add the eggs one at a time, beating well after each addition.
6) Add the dry ingredients in three parts, alternating with the milk and vanilla. Scrape the sides and bottom of the bowl, and beat for 1 more minute.
7) Divide the dough among the prepared pans. If you're using a scale, put 1 1/2 pounds of batter in each pan.
8) Bake the cakes for 35 to 40 minutes, or until a cake tester or toothpick inserted in the center comes out clean. Remove from the oven and cool in the pans for 15 minutes. Turn the cakes out of the pans and cool completely on a rack.
9) To make the icing: Beat the butter with 4 cups of the sugar. Alternately beat in the milk and vanilla with the remaining sugar, adding enough sugar to make icing with a nicely spreadable consistency.
10) To assemble the cake: Trim any domes off the tops of the layers. Place a layer on a serving plate lined with parchment paper strips.
11) Spread the layer with frosting, and top with half the sliced strawberries and blueberries. Spread the trimmed top of the next layer with 1/4 cup of the strawberry preserves, and place it atop the berries. Repeat with the remaining layer.
12) Frost the top and sides of the cake with the remaining icing.
Yield: 20 servings.