Preheat oven to 350 degrees F. Rinse and dry blueberries. Lightly grease inside of mason jars with butter wrappers.
Cut french bread into small 1" squares and divide between mason jars. Fill jars to the top and set aside.
In a mixing bowl combine milk, cream, eggs, sugar, and vanilla. Beat until ingredients are combined. Next, dollop blueberry jam over bread in each jar. Pour cream mixture over bread and divide between jars so each gets an equal amount. Set aside for bread to absorb mixture.
Slice butter into thin slivers and top the jars with butter slices. Divide blueberries and cover each jar with berries.
Lastly, set jars on a cookie sheet and place in the oven for 30 minutes or until liquid is a custard consistency. Let jars cool before serving. Sprinkle with fresh blueberries and powdered sugar and enjoy!