Salt (Sodium chloride) is salt. As a topping, flakes are commonly used purely for presentation purposes only. The taste is the same, but gets more intense as the salt particles get finer, so use less if the salt is in powder form
As an ingredient, use any form you are happy with, and is economical to use. Once salt is dissolved into water it will be identical to any other form of salt
For health reasons, finely powdered salt is preferable as much less is required to impart a salty taste
If you want a sculpted cake of this type:
then I would advise against trying it at home. This is a skill you have to learn over years. Basically, you have to be an experienced sculptor in working with cake materials, which are harder to use than the typical art materials like clay. Look up "cake sculpting" on YouTube to see what they are doing. Or search for Cake boss episodes, taking into account that this "reality" TV is heavily edited - they are showing you a very clean and quickened version of the process. Really, I think that 500 dollars is cheap for having your drawing sculpted. By the way, the airbrushing tool alone used for finishing the cake in the picture runs at around 200 dollars, although of course you don't strictly need it for making the cake in such a shape.
You seem to be good at drawing. If you can make a finished, colored drawing out of it instead of just a sketch, you can go to a bakery who has a foodgrade printer, and have it printed on the cake. They mainly advertise it as "your cake with your photo on it", but of course they can print a digitized drawing too.
If you don't want a picture, it is also doable as a 2d picture piped onto a flat cake. If the 500 dollars are for a sculpted cake, then having the picture piped can be much cheaper (I don't think that they would have required 500 dollars just for piping, but who knows, maybe you live in Manhattan). It would be this type of cake, but with your drawing on it:
If you have steady hands and some artistic experience, this kind of decoration is a skill that can be learned in maybe 20 hours. Take a piping bag, make icing, and practice on other surfaces first, even stretched plastic wrap. When you are good enough, attempt the drawing on a practice surface. Then you are ready to make the cake. Fondant or marzipan will look best as the background for the piping, but they are very hard to work with. If you have never done it before, use a cake frosted with buttercream as the background. If you have the time and inclination, you can learn how to make marzipan flowers and add them as 3-d elements on top of your drawing.
The basic skills you need are:
- bake consistently good bisquit layers. You can cut off domed parts, but it is a hassle, and if you are not good at getting the leavening right, you are risking cutting into a metallic tasting cake after all this work.
- Making buttercream. It is not hard, but requires a bit of exercise and a few tools. You should be accustomed to baking by weight, know the feel of butter and eggs at the right temperature and the right stage of beating, etc. If you have never done it before, again, make 2-3 batches of buttercream before you try the actual birthday cake.
- Frosting cakes. Your drawing will probably require a large rectangular cake, which is harder to get evenly frosted than a small round one on a turntable.
- I already covered the piping. Marzipan flowers are a further skill I mentioned, but optional.
The Internet is full of tutorials on decorating cakes, watch YouTube videos about the more visual parts. There are good articles on making icing too, and we also have one or two older questions on that.
Best Answer
There are two ways to get the shape. One way is to bake them spherical, the other to crumble the cake and to roll the crumples with icing.
To bake them spherical, you need a mold that goes into the oven, or an appliance for the task.
Your basic cake recipes are what you want here, nothing fancy. White cake, yellow cake or chocolate cake, there's no reason to make it more complicated, considering that your presentation is going to be novel anyway.
A simple buttercream frosting would be fine for rolling the balls (if that's the way you choose to go), but you would want more of candy shell on the outside. Sure, Wilton brand candy melts are an option, but you might find almond bark easier to find. That comes in chocolate and white, and will set more like the candy melts, less soft and melty than real chocolate would be. White bark could be colored how ever you want (use gel coloring, not liquid). Here's an article you might find helpful about better choices than real chocolate for this kind of shell: Love from the Oven
The sweetness of your cake and icing is totally up to you. One thing to consider instead of reducing the sugar would be to possibly add a bitter element, like lemon zest.
The best thing you can use for the sticks, if you can find them. would be the plain paper ones: