In a large mixing bowl, cream butter and sugars until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla.
Combine flour, baking powder, cinnamon, and nutmeg; gradually add to the creamed mixture. Stir in zucchini and optional extras. Spread into a greased jelly roll (15x10x1-inch) baking pan.
Bake at 350°F for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack.
For icing, combine confectioners' sugar and cinnamon in a bowl. Stir in the butter, vanilla, and enough milk to achieve a thin consistency. Drizzle icing over bars before cutting.