Leave bread slices out, at room temperature, at least 3 hours, or until slightly stale. Generously coat 9-inch baking dish with nonstick cooking spray. Arrange bread, slightly overlapping.
In large bowl, whisk eggs, heavy cream, milk, Gruyere cheese, maple syrup and cinnamon. Add chopped bacon, saving 2 tablespoons for later. Pour over bread in baking dish.
With hands, press down on bread to submerge. Cover baking dish with plastic wrap. Refrigerate at least 5 hours, or overnight.
Heat oven to 375 degrees when ready to bake. Remove plastic. Sprinkle remaining bacon and Swiss cheese over top of bread mixture.
Bake 45-50 minutes, or until golden brown and set. Top with blackberries and dust with powdered sugar before serving.