Preheat oven to 375 F (190 C). Lightly grease 2 muffin pans, or use paper liners.
In a large bowl, mix together flour, baking powder, 2 teaspoons cinnamon, 1/4 teaspoon nutmeg, allspice and salt. Mix in brown sugar, white sugar and coconut. In a separate bowl, combine eggs, oil, 1/2 cup buttermilk, carrots, pineapple, and vanilla. Make a well in the middle of the flour mixture and add the egg/buttermilk mixture. Mix until batter is moistened. Fill muffin cups 3/4 full.
Bake in preheated oven for 20 to 25 minutes, or until the tops spring back when lightly tapped. Allow to cool.
In a small bowl, combine confectioners' sugar, 1 teaspoon cinnamon and 2 to 3 tablespoons buttermilk. Mix until smooth, then refrigerate until thickened. Drizzle over cooled muffins. Sprinkle with chopped nuts, if desired.