I want to make a unicorn cake and was going to make it four layers tall - probably 6 inches diameter. Do I need to do two layers and then have a cake board / dowels before the other two layers? Like I would normally with a stacked cake. I have seen quite a lot of unicorn cakes that don't do this?
Also, do I just use normal buttercream for the outside of the unicorn - I don't need to colour it to make it more 'white' in any way?
Separating the layers with a cake board will provide you with double the servings, while not separating them will give you very tall servings. So it really depends on the number of servings you need vs the type of slice you are going for. As for the buttercream again, it is really the look you are going for. If you want to get the buttercream whiter then you could substitute some of the butter for vegetable shortening or just use all shortening and clear vanilla extract which will give you a pure white frosting. If you want to stick with all butter, buttercream you could also tint your buttercream with a very, very tiny amount of purple gel food coloring (like just a tiny bit on the tip of a toothpick). By adding the purple it will help "remove" the yellow that you get from the butter.
http://www.pinkpistachio.com/birthday-cake/ (this lady adds the purple when creaming the butter
http://shewhobakes.co.uk/how-to-make-white-buttercream/ (this lady adds the purple when the buttercream is complete)
Yes, Freckles is right. YOu would be better off putting the cake board between the two tiers - lightly doweling the lower tier. (All you would need are 3 dowels placed in a triangle fashion.) Remember that very, very few dessert dishes will hold a slice of cake over 4" tall.
I always used my b'cream - made w/half butter w/o changing it - even for wedding cakes. I see no need to 'whiten' white b'cream - especially if you use this recipe: https://www.cakecentral.com/recipe/22469/2-icing