Butter a 9” x 13” baking dish. Arrange the torn challah to cover the baking dish completely, leaving no gaps in between the bread. Nestle the sliced butter and raisins in between the bread.
In a large bowl, whisk together eggs, milk, cream, ground piloncillo, cinnamon, vanilla, and salt until well combined. Using a ladle, pour the milk mixture over the bread to coat each piece very well. Cover and refrigerate overnight.
About an hour before serving, remove cover and sprinkle the pecans evenly over the top. Bake at 350 degrees Fahrenheit until golden and puffed, about 45 to 60 minutes. Let stand for about 10 minutes before serving.
To serve, dust with powdered sugar and cut into squares before serving with maple syrup and fresh berries.