Preheat the oven to 325 degrees F. Grease a 13-inch by 9-inch baking pan with butter, line with parchment and butter again.
In a medium bowl, combine the flour, cornmeal, baking powder and salt. Using an electric mixer, cream the butter and sugar on medium high speed in a large bowl, until light and fluffy, about 2-3 minutes. Beat in the eggs one at a time followed by the vanilla extract. On low speed, mix in the dry ingredients in 3 additions alternating with the buttermilk in 2 additions until just combined.
Spread the batter into the prepared cake pan. Sprinkle with the berries and the remaining sugar on top. Bake the cake until golden brown and a toothpick inserted into the cake comes out clean, about 1 hour. Cool the cake completely on a wire rack. Dust with confectioners’ sugar before serving.