Preheat oven to 350ºF. Butter two 4"x2" round cake pans and line bottoms with parchment rounds. Dust sides with flour and tap out excess. Line 3 cups in a muffin tin with cupcake liners.
In a large bowl, whisk together the flour, vanilla bean seeds, baking powder, baking soda, and salt.
In the bowl of a stand mixer fitted with the wire whip attachment (or using a second large bowl and handheld electric mixer) begin beating the egg whites on medium-high speed. After a few seconds, slowly pour in the sugar while continuing to beat. Beat for 3-4 minutes, or until white and fluffy.
Reduce mixer speed to medium. Slowly and carefully pour melted butter into egg mixture while beating. Add vanilla extract and continue to beat just until all ingredients are incorporated, wiping down bowl and beater as needed.
With mixer on low speed, add 1/3 of the flour mixture followed by 1/2 of the buttermilk. Continue to alternate adding dry and wet ingredients in this manner until all have been added, beginning and ending with dry ingredients. Mix just until evenly incorporated, again wiping down bowl and beater as needed.
Remove half the batter from the mixer. Add the buttermilk and cocoa powder for the chocolate marble and mix in on lowest speed until mostly incorporated. Finish folding in by hand.
Alternate adding white and chocolate batter to cake pans, filling each about 3/4 full. Repeat with remaining batter to fill cupcake liners. Swirl batter briefly with a knife or icing spatula to achieve marble effect.
Bake cakes/cupcakes in preheated oven until a toothpick inserted into the center of each comes out clean or with just a few moist crumbs, about 20 minutes for cupcakes and about 40 minutes for cakes. Allow cupcakes to cool in pan for a few minutes before moving to a wire rack. Let cakes cool completely in pans on a wire rack before removing.
Once cool, wrap cakes tightly in plastic wrap. Freeze for 30 minutes prior to frosting. Cupcakes can be kept in an airtight container at room temperature until ready to frost.