Preheat oven to 325 degrees. Grease and flour a tube pan or bundt pan.
Cream butter and shortening for 3-5 minutes using an electric mixer with a paddle attachment (or a hand mixer). Gradually add sugar.
Add eggs one at a time, stopping to scrape down sides of bowl.
Alternate between adding flour and Fanta, beginning and ending with flour.
Beat in vanilla extract, orange zest, salt, and whipped topping mix.
If desired, add food coloring to get a deeper orange color.
Pour batter into pan and bake for approximately 1 hour and 15 minutes.
Let cool in pan for 15 to 20 minutes and then invert onto a cake stand.
To make Icing, beat butter and shortening together until smooth. Add orange juice. Gradually beat in confectioners' sugar and milk. If too thick, add a little more milk or orange juice.
Spread icing on top of the cake and let it spread down the sides some.
Notes
You want to beat the butter and shortening together really well before adding sugar and then continue to beat until fluffy and very light in color. Once eggs are added, beat as little as possible.