Preheat oven to 300 degrees F. Arrange bread cubes in a large shallow baking pan. Bake about 15 minutes or until dry, tossing once. Cool completely.
Grease a 3-quart rectangular baking dish; set aside. In a large mixing bowl beat 8 ounces mascarpone cheese and butter with an electric mixer on medium to high speed for 30 seconds. Add sugar and vanilla. Beat until combined, scraping sides of bowl occasionally. Add eggs, one at a time, beating well after each addition. Gradually add half-and-half and milk, beating on low speed until combined.
Pour cheese mixture into the prepared baking dish, spreading evenly. Top with half of the bread cubes, all of the fruitcake cubes, and the remaining bread cubes. Using the back of a large spoon, gently press down on layers. Cover with plastic wrap and chill for 4 to 24 hours.
Preheat oven to 350 degrees F. Remove plastic wrap. Bake for 40 to 45 minutes or until set and golden. Cool on a wire rack for 30 minutes. Serve warm. If desired, sprinkle with powdered sugar and top with additional mascarpone cheese.