Preheat oven to 350 degrees. Lightly grease two 9-inch-round cake pans with butter.
Place butter in a large bowl and stir until creamy. Add honey and molasses, mix until smooth.
Sift flour into a large bowl. Stir in ginger, arrow root powder, baking soda, cinnamon, nutmeg, and mace.
Zest the orange. Cut orange in half and squeeze juice to yield about 4 tablespoons. Add it to the zest in a small bowl.
Fold in flour mixture along with warm milk, grand marnier, eggs, and orange juice and zest. Mix until smooth. Fold in chopped walnuts.
Fill prepared pans with batter. Bake 35 to 40 minutes or until a toothpick inserted in the center comes out clean.
Allow to cool in the pan for 15 minutes, then slice and serve with powdered sugar or whipped cream topping, if desired.