Preheat oven to 350 degrees. Spray a bundt pan with cooking spray and set aside.
Peel your zucchini and cut into chunks (remove any seeds). Place the chunks into a blender or food processor and blend. You will probably have to stop it a few times and give it a stir before it turns into a puree.
In a separate bowl combine the melted butter, milk, sugar, eggs and zucchini. Add this to the dry ingredients and mix on medium speed until smooth.
Pour the batter evenly into prepared pan. Bake at 35 to 40 minutes or until a knife or toothpick inserted in the center of the cake comes out clean.
In a medium sized mixing bowl combine butter and sour cream, mix with an electric mixer until smooth. Add powdered sugar, vanilla and salt and continue mixing until smooth. Spread (or pour) over cake.
I usually throw an extra tablespoon or two of sour cream in just because I love the flavor it adds to the frosting. I also like to make my frosting thicker, so it's a spreadable consistency. You can keep yours thinner, more pourable if you'd like. It's definitely a personal preference here.