How to Make It
Preheat oven to 350°F. Line the bottom and sides of a lightly greased 13- x 9-inch pan with parchment paper. Beat butter and sugar with an electric mixer at medium speed until light and fluffy, 3 to 4 minutes, stopping to scrape sides of bowl as needed. Add eggs, 1 at a time, beating well after each addition and stopping to scrape sides of bowl as needed.
Whisk together the flour, cornmeal, baking powder, and salt in a medium bowl. Whisk together the sour cream, orange zest, orange juice, and vanilla in a separate bowl. Add flour mixture to butter mixture alternating with sour cream mixture, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Spread batter into prepared baking pan, and scatter berries and plum slices over batter.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 55 minutes to 1 hour and 5 minutes. Cool on a wire rack 20 minutes. Cut into squares, and serve warm or at room temperature.