Cut the cinnamon loaf into 1-inch pieces. Grease a 2-quart baking dish using 1 tablespoon of the butter.
In a large bowl, beat the eggs, milk, 1/2 cup of the sour cream, the granulated sugar, cinnamon and salt with wire whisk until well mixed. Fold in the bread cubes.
Pour the mixture into the baking dish. Cover and refrigerate for 4 hours or overnight.
Heat the oven to 400 degrees. Uncover the baking dish; bake for 20 minutes or until set and a knife inserted in the center comes out clean. Cool for 5 minutes.
Meanwhile, in a medium bowl, beat the remaining 1/4 cup of sour cream, remaining 2 tablespoons of butter, powdered sugar and vanilla with a whisk until smooth. Spoon and spread over the baked dessert.