Preheat the oven to 350 F. Butter two 9" round cake pans. Line bottoms with parchment circles and butter the parchment. Set aside.
Whisk together the cake flour, baking powder, and salt. Set aside.
Whisk together buttermilk, egg whites, vanilla extract, and the vanilla bean seeds in a medium bowl. Set aside.
In mixer fitted with paddle attachment, beat butter and vanilla sugar until fluffy, about 3 minutes. Reduce speed to low and add 1/3 of the dry ingredients, then 1/3 of the wet ingredients. Continue alternating until homogenous. Beat 2 minutes more on medium speed to aerate the batter.
Pour batter into prepared pans. Place mulberries on top of batter in each pan.
Bake for 30-35 minutes, cake edges will begin to brown and pull away from the edge of the pan. Cool in pan on wire rack for about 15 minutes. Remove from pans, peel off parchment, and let cool completely.
Syrup
Mash the berries with a potato masher. Push the berries through a strainer by pressing with the back of a spoon. You should end up with approximately one cup of juice (and any pulp that makes it through). Put the juice and pulp into a medium saucepan. Add the sugar and vanilla and cook on medium high, stirring occasionally, for 5 minutes. Turn heat down to medium low and cook for another 5 minutes. Remove from heat and let cool to room temperature. You should end up with approximately 1/3 cup of thickened syrup.
Frosting
In mixer fitted with paddle attachment, beat the cream cheese, powdered sugar and vanilla until smooth and creamy. Thoroughly clean mixer bowl and fill with ice water to chill the bowl. Let sit with ice water for a few minutes to get cold. Fit mixer with the whisk attachment. Drain out the ice water and wipe dry. In the chilled mixing bowl, beat the whipping cream on medium-high until stiff peaks are formed. Gently fold into the cream cheese mixture. When most of the cream cheese and whipped cream are incorporated, whisk the ingredients together to make smooth.
Assembly
Reserve 2 tablespoons of the syrup and set aside. Spoon about half of the remaining syrup onto the first cake layer, spreading to within an inch of the edge. Add one cup of the frosting to the top of the syrup. With a light hand, using an icing spatula, carefully cover the jam with the frosting.
Place the second cake layer on top of the first and carefully press down and twist a little to set. Repeat the same process with the syrup and frosting as with the first layer. Use remaining frosting on sides and add more to the top as well.
Drizzle the reserved syrup on top of the cake, then swirl an icing spatula through the syrup to add the marble effect to the top of the cake. Garnish with a few fresh mulberries. Refrigerate cake for 2 hours before serving. Cake should be refrigerated when not being served.