Whisk the flour, polenta, baking powder, and salt in a medium bowl. Cream together the sugar and butter until fluffy, in a stand mixer or handheld mixer. Add the vanilla and zest and beat to combine. Add the eggs one at a time, beating well after each addition. Scrape the bottom of the bowl a couple of times to make sure everything is incorporated. Add the flour mixture alternately with the yogurt, beating on low speed after each addition just until incorporated. Begin with flour mixture and end with flour mixture. Fold the fresh cranberries with a spatula, do not over mix. Spread the thick batter evenly in the prepared pan. Bake for 40-45 minutes, until toothpick inserted into middle comes out clean.