For the polenta: Bring the broth, 2 1/2 cups water and the salt to a boil in a heavy 5-quart saucepan over medium-high heat. Gradually whisk in the polenta. Reduce the heat to low and cook, stirring often, until the mixture thickens and the cornmeal is tender, 15 to 20 minutes. Remove the pan from the heat. Add the Parmesan, milk and butter. Stir until the butter and Parmesan have melted. Season with salt and pepper.
For assembly: Place an oven rack in the center of the oven. Preheat the oven to 400 degrees F. Arrange the bacon in a single layer on a rimmed baking sheet. Bake until brown and crispy, 15 to 17 minutes. Drain on paper towels. When the bacon is cool enough to handle, chop into 1/2-inch pieces. Set aside.
Whisk together the mascarpone cheese, maple syrup and salt in a small bowl until smooth.
Transfer the polenta to 4 bowls or serving plates. Divide the bacon pieces and spoon on top. Dollop each serving with the mascarpone mixture and sprinkle with the dried cranberries.