These cookies are a great way to not throw away a leftover morning cereal that you may have left. Instead of granola or muesli you can also use cereal flakes (like Corn Flakes®) or puffed rice grains (like Rice Krispies®)
It is difficult to give precise doses for these croquettes precisely because, being prepared with leftovers, the quantities can change. The advice I can give you, therefore, is to taste the mixture and test its consistency.
If you have little time, you can pour the mix into a cake pan that goes into the microwave and cook the cake at 600W for about 13 minutes. After this time, open the microwave oven, check if the cake has thickened, otherwise cook for another couple of minutes. If you have a microwave that reaches 800W, you can cook it for only 10 minutes.
In the Christmas season I often prepare the panettone in which only yolks are needed: this is a very good and very sweet way to use egg whites without throwing them away!
The ideal panzanella bread is the thick soft type like Tuscan or Puglia’s bread. The choice of bread is important: if you choose one such as baguette, when you wet it, the water will emerge on the surface of the bowl without softening adequately the bread.