Preheat oven to 350ºF.

Wash and remove grapes from stem.

If grapes are large, coarsely chop, roughly quartering them and reserve.

In a large bowl, combine flour, baking powder and salt.

In another bowl, whisk together sugar and olive oil until sugar is dissolved and mixture is fluffy.

Whisk yogurt, eggs, rosemary and vanilla into the sugar mixture until well combined.

Mix flour mixture into yogurt mixture and whisk until combined. Reserve.

Using a 9-inch cast iron skillet, heat over medium-high heat and melt butter.

When butter is bubbly, stir in brown sugar.

Cook until sugar has dissolved into the butter and starts to melt.

Stir in the grapes and cook until softened, about 3 minutes. Gently press down on the grapes with a spatula to release some of the juices.

Stir in balsamic vinegar and cornstarch with the grapes.

Remove skillet from heat. Slowly pour batter over the grapes, using a rubber scraper to spread the batter evenly.

Place cake in the oven and cook for 30 minutes, or until a toothpick inserted into the center of the cake comes out clean. The top of the cake will be lightly browned.

Allow cake to cool for 15 minutes in the pan.

Run a knife blade along the sides of the cake to insure a clean removal. Place cooling rack over the cast iron skillet and carefully invert the cake. Keep the skillet over the cake until the cake cools, about 1 hour.

Slowly remove skillet and allow cake to finish cooling to room temperature, about 30 more minutes, before serving.