products variants quantities

My question was about the possibility of having several toppings for one cake.

From your past examples you don't mention that possibility, so I wondering why you'd want a quantity box for sauces ?

Suppose that you have a product with two characteristics: sauce and frosting:

Cake

£5 per unit
frosting: (dropdown with the different frostings)
sauce: (dropdown with the different sauces)
(qty box) (add to cart button)

Suppose that you want to buy 12 cakes with a choice of vanilla frosting, 4 with red sauce, 4 with blue sauce and 4 with white sauce which is the example you gave earlier.
You select the vanilla frosting and the red sauce, set the quantity to 4 and click on (add to cart). Then, you change the sauce to blue and click again on add to cart. Finally, you change again the sauce to white and click again on add to cart.
You now have 12 cakes in your cart, 4 per sauce type, and you can proceed to the checkout.

In that scenario, I don't see why you would want a quantity box for sauces since each sauce selection is "attached" to one cake.