In a medium bowl, combine the berries, jam and a pinch of salt. Gently toss, then let stand at room temperature until the berries soften and release juices, about 30 minutes, or cover and refrigerate up to 12 hours. Meanwhile, in a large bowl and using a silicone spatula, stir the mascarpone to soften. Stir in the sugar. Whisk in the cream and vanilla until it holds soft peaks; do not overwhip. Cover and refrigerate until ready to use or up to 12 hours.
Set 1 of the cakes on a platter. Spoon on half of the berry mixture, leaving a 1-inch border at the edge. If the mascarpone cream has been refrigerated for more than a couple of hours, give it a stir to re-soften. Dollop half of the mascarpone cream over the berries then, using the back of a large spoon, spread evenly, leaving a 1-inch border at the edge.
Place the second cake on top. Scoop the remaining cream onto the center of the cake. Using the back of the spoon, spread the cream, leaving a 1-inch border at the edge and creating swooshes, swirls and a shallow well in the center. Spoon the remaining berry mixture into the center and drizzle with the juices remaining in the bowl. Serve right away.
Cake
Heat the oven to 350°F with a rack in the middle position. Mist two 8- or 9-inch round cake pans with cooking spray. Line the bottoms with rounds of kitchen parchment, then mist the parchment. Dust the pans with flour, then knock out the excess.
In a large bowl, whisk together the flour, baking powder, baking soda and salt. In a blender, combine the buttermilk, butter, whole eggs and egg yolks, oil, vanilla and sugar. Blend on high just until combined, about 15 seconds, scraping the jar about halfway through to make sure the sugar has not clumped on the sides; do not overblend. Pour the wet ingredients into the dry ingredients, then whisk just until evenly moistened. Divide the batter evenly among the prepared pans, then rap each pan 2 or 3 times on the counter to remove any air bubbles.
Place the pans in the oven, evenly spaced on the rack. Bake until golden brown and a toothpick inserted in the centers comes out clean, 22 to 25 minutes. Cool in the pans on a wire rack for 15 minutes. Run a paring knife around the edge of each cake to loosen. Invert the onto the rack, lift off the pan and peel off the parchment. Cool completely, about 1 hour.
