1. Preheat oven to 325 degree F. Butter two 9- by-5-inch loaf pans. Line bottoms of pans with waxed paper, then butter paper and flour pans, tapping out excess flour. Whisk flour, seeds, and baking powder together in a medium bowl. Whisk eggs, milk, and 1-1/2 teaspoons vanilla together in another bowl.
2. Beat butter, 2 cups granulated sugar, the zest, and salt together in a large bowl with an electric mixer on high speed until light and fluffy, about 6 minutes. With mixer on low, alternately add flour and egg mixtures in batches, scraping bowl as needed, until combined.
3. Divide batter between prepared pans, smoothing tops. Bake in middle of oven until tops are golden brown and a toothpick inserted in center comes out clean, about 1 hour. Cool in pans 10 minutes. Run a thin knife around edges and invert cakes onto a rack to cool.
4. Meanwhile, toss berries in a bowl with 2 tablespoons granulated sugar and the juice. Let stand, stirring occasionally, 15 to 25 minutes.
5. Beat heavy cream with remaining 2 tablespoons granulated sugar and 1 teaspoon vanilla in a large bowl with an electric mixer on medium speed to soft peaks.
6. Cut each cake into 3 horizontal layers. Divide and spread 3/4 berry mixture and all of whipped cream between layers to form 2 layered cakes. Top each with remaining berries and dust with confectioners' sugar. Makes 12 servings (2 cakes).