Preheat oven to 350 degrees and generously spray a 9 x 13" baking dish with nonstick spray or butter well.
Cream together the softened butter and brown sugar until light and creamy. Add the eggs one by one, beating well after each addition. Add the vanilla extract.
In another bowl, whisk together the flour, baking powder, baking soda and salt.
Alternate adding the dry ingredients with the buttermilk to the creamed butter and sugar mixture. Mix batter until only combined then spread it out in your prepared pan.
Bake for 20-25 minutes until golden brown. Let cool completely while you prepare the frosting.
To make the frosting, whip the COLD heavy cream with the powdered sugar and vanilla on high until peaks form. Spread on the cold cake (if cake is still warm, frosting will melt and run all over the place). Decorate with fresh berries and eat immediately.