In a large mixing bowl, add the eggs and sugar, and cream together until light and fluffy.
Add the brown sugar, orange zest, canned pumpkin, oil, and vanilla, and blend well.
In the middle of the pumpkin/egg mixture, add the flour, baking powder, baking soda, cinnamon, allspice, and salt, and blend the dry ingredients together right on top of the pumpkin mixture, and then blend the flour and spices into the pumpkin mixture until well blended
Set mixture aside, and butter the bottom and sides of two loaf pans generously with butter, and then flour the pans, tapping out any excess flour.
Evenly divide the batter between the two pans, and spread the batter out with a spoon.
Place the pans in the oven at 350 degrees, and bake for about 50 minutes to one hour, or until a toothpick inserted in the center comes out clean. When done, remove to wire racks to cool.