Now those would be quite large ice "cubes". Probably too large. However I have plans of my own that may seem familiar.
Firstly, muffins are required.
But these muffins are probably evil muffins and need to be destroyed by tearing into pieces and squishing into your select mould. The remains are then boiled in liquified jelly crystals for safety (it probably stops necromancers from reanimating the evil muffins. To be extra safe you will also refrigerate these for a number of hours.
Now because my mould is *supposed* to be for ice cubes it is a soft plastic, maybe silicone. Now for ice cubes that is good because the ice cubes would be solid but for jellymuffins you'll totally destroy the jellymuffin when you push it out of the mould. My solution is to chuck this into the freezer for a sufficient length of time to freeze the jelly and allow you to push it out without ruining it. If you do it this way too you'll need to make sure the thawed brains have drained as much ice away as possible. Here are some mini-brains thawing out.
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