Preheat oven to 350 degrees. Grease a 9-inch springform pan with cooking spray or butter and flour; set aside.
Whisk together the flour, baking powder, and salt; set aside.
In a large bowl, use a handheld mixer to beat together the eggs and sugar until the mixer is thick; about 3-4 minutes. Add in the milk, oil, butter, orange zest, and cinnamon, and mix until combined. Mix in the flour mixture until just combined. Fold in the purple grapes. Pour the mixture into the prepared pan.
Bake on the center rack of the oven for 50-60 minutes or until a toothpick inserted in the cake center comes out clean. Let the cake cool in the pan for 10 minutes. Run a knife around the edge, the release and remove the sides of the pan. Let cool completely. Sprinkle with powdered sugar before serving.
Video
Notes
If you want more of a citrus taste to your cake, you can add additional orange and lemon zest to the cake batter.
Traditionally this cake is made without cinnamon. Feel free to leave it out (or add more) depending on your tastes.
Small grapes are key here. Try to find the smallest grapes you can. You can make this with bigger grapes if that's all you have. Don't cut the grapes to make them smaller; it will just make the batter too wet.
Store this cake in an airtight container in the refrigerator for up to a week. Or you can freeze for up to a month.