Preheat your oven to 375F. Butter your baking dish with melted butter.
Arrange the grapes at the bottom of the pan.
Put all the remaining ingredients - milk, eggs, butter, sugar, vanilla, and flour into the blender. Secure the lid and blend on high for 60 seconds. Scrape down the side to make sure there are no clumps in the batter.
Carefully pour the blended batter over the prepared dish.
Gently tap the dish to settle the grapes and mixture. Place the filled dish onto a cookie sheet (to catch any spillage) and put the whole thing into the oven.
Bake in over for 50 minutes. Check to make sure the center is set. If not, bake for another 10-15 minutes.
Remove from oven. Cool, and dust with icing sugar if desired. Slice and enjoy!