Make the cake: Position a rack in the center of the oven and heat the oven to 350 degrees F. Grease the bottom and sides of two 9-inch round cake pans. Line the bottoms with parchment and lightly flour the sides of the pans, tapping out any excess.

Sift the cake flour, baking powder, and salt onto a paper plate or into a medium bowl. In a large bowl, beat the butter with a stand mixer fitted with the paddle attachment (or a hand-held electric mixer) on medium speed until smooth, 1 minute. Add 1-1/2 cups of the sugar and the vanilla bean seeds or extract. Continue beating until well combined and fluffy, 2 minutes. Stop to scrape the bowl as needed. On low speed, add the one-third of the dry ingredients at a time, alternating with 1/2 cup of the milk at a time, beginning and ending with the flour. After the last addition, scrape the bowl and mix for about 30 seconds to mix the batter fully.

In a medium bowl, beat the egg whites with an electric mixer (a stand mixer fitted with the whisk attachment or a hand-held) on medium-high speed until soft peaks form. Increase the speed to high and gradually add the remaining 1/4 cup sugar. Continue beating until the whites form medium-firm peaks. Using a rubber spatula, scoop up about one-quarter of the whites and stir them gently into the cake batter to lighten it. Gently fold in the remaining whites until just blended.

Scrape the batter evenly into the prepared pans. Bake until the tops are light brown and a toothpick or cake tester inserted in the center of the cake comes out clean, about 30 minutes. Set the pans on a rack and let cool for about 15 minutes. Run a knife between the cake and the pan to loosen each cake. Invert the layers onto a rack, lift off the pans and peel away the parchment. Let cool completely.

Make the frosting: In a medium bowl, combine the mascarpone, cream, sugar, vanilla seeds or extract, and salt. Using an electric mixer, beat on low speed until almost smooth, 30 to 60 seconds. Increase the speed to medium high and beat until the mixture is thick and holds firm peaks, another 30 to 60 seconds. Don't overbeat or the frosting will look grainy.

Assemble the cake: Using your hands, gently brush away any excess crumbs from the layers. Set one cake layer, top side down, on a flat serving plate. To protect the plate from smears, slide small strips of foil or parchment under the bottom of the cake to cover the plate. Using a metal spatula or the back edge of a table knife, spread about 2 cups of the frosting evenly over the layer. Arrange about half the berries in a single layer on the frosting but leave a half-inch ring of space around the edge of the cake uncovered. Place the second cake layer, top side down, on top of the frosting. Be sure the sides are aligned and then press gently on the layer. Apply a very thin layer of frosting over the entire cake to seal in any stray crumbs. Chill in the refrigerator for 5 minutes. Spread the remaining frosting over the top and sides of the cake, leaving lots of swirls and peaks on the top. Garnish the top with remaining berries. Carefully remove the foil or parchment strips from under the cake. Refrigerate the cake for 4 hours or up to two days. To keep the fruit looking fresh, cover the cake loosely with plastic after it has chilled for 1 hour.