Grease and flour a 9" cake or spring form pan. I recommend lining the bottom with parchment.
Whisk the dry ingredients together so they are well mixed. Set aside.
Cream the butter and sugar together until light and fluffy. Beat in the eggs, one at a time.
Add the vanilla extract and the seeds from the bean.
Add the flour to the bowl, alternately with the buttermilk, beginning and ending with the dry ingredients. Beat just until combined.
Turn the batter into the pan.and smooth it out evenly.
Bake for about 30-35 minutes, until a toothpick comes out without wet batter clinging to it.
Cool on a rack completely before frosting.
To make the frosting: Wash and hull the berries. Cut them in half and put them in the bowl of a small 4 cup food processor. Process until you have a smooth puree. Scrape down the sides of the machine as necessary.
Cream the butter with 2 cups of the sugar. ,Add just enough puree to form a spreadable frosting. Beat until smooth.and adjust the texture with more sugar if too wet, or more puree if too stiff.