In a large mixing bowl combine flour, cocoa, baking soda, and salt.

In a separate bowl combine shredded zucchini, sugar, and vegetable oil. Add to dry ingredients, stirring by hand. Stir in vanilla.

Pour batter into a greased 9x13 inch pan and bake at 350°F for 30-40 minutes or until a toothpick inserted into the center comes out clean.

Cool pan on baking rack for 30 minutes.

Melt butter in a small saucepan over medium-high heat. Add in sugar and milk. Bring to a boil, stirring often.

Reduce heat to medium and continue to cook for 1 minute stirring often.

Remove from heat; stir in chocolate chips and marshmallows until completely melted. Stir in vanilla.

Let cool, stirring occasionally, until frosting thickens slightly.

Pour over cake and spread until smooth.