Heat oven to 350°. Coat a 9x2-inch square baking pan or 2-quart square baking dish with nonstick cooking spray.
In a large bowl, toss bread with 3/4 cup of the mini chips. In a medium bowl, whisk together chocolate milk, eggs, brown sugar and vanilla.
Pour milk mixture into bread mixture, then transfer to prepared baking pan, pressing down lightly. Sprinkle with remaining 1/4 cup mini chips.
Bake at 350° for 45 minutes. Dust with confectioners' sugar just before serving.