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.