Preheat oven to 375 degrees. Grease a deep dish pie plate with the butter, then add the berries and set aside.
In a large bowl, combine the flour, sugar, and salt. Whisk in the eggs, then add the milk, cream, and vanilla and stir until smooth.
Pour the mixture gently into the pie plate over the berries, place in the oven, and bake for 25 minutes, or until the custard has puffed and is set, but still slightly jiggly in the middle.
Allow to cool on a rack for 15 minutes, sprinkle with powdered sugar, then enjoy!