Beat softened butter at medium speed until creamy. Gradually add the 1 cup granulated sugar, mixing until light and fluffy. Add eggs, 1 at a time, beating just until incorporated after each addition.

In a medium bowl, whisk together the flour, baking powder and salt. Add mixture to the butter mixture alternately with the milk, mixing at low speed just until blended, beginning and ending with the flour mixture.

In a small bowl, mix together the lemon and lime zest. Add 1 tablespoon of the zest mixture to the batter, reserving the remaining tablespoon for the zest topping.

Spoon the prepared batter into a greased and floured 8x4-inch loaf pan. Bake at 350 for 1 hour, or until a wooden skewer inserted into the center comes out clean. Cool in pan for 10 minutes before removing to a wire rack to cool completely.

For the glaze, in a medium bowl, combine the powdered sugar, lemon and lime juice and vanilla extract, whisking well to combine. Drizzle over the cooled loaf. Combine the remaining lemon and lime zest with the 1 tablespoon granulated sugar and the flaked coconut, if using. Sprinkle on top of the bread.