Is the right image the product of the last one ?
It’s difficult for me to understand the final form of the jello you want to get … And that’s because I see those cylinders protrude like a crown. This can’t be the final form of the jello, it will brake during the unmolding.
And the base looks thin, it may hold a hard cake but not a jello… (we are talking about a real mold right ?)
From what I understand is that you want to have the left model inside the right one and the final to be hollow, right ?
Take the left one and make a copy. Move the clones a bit towards the center of the array and a bit down and use the copy to subtract it from the original. Then just combine under a union the left and right.
(well in my opinion due to the low thickness of the mold it would be better to have the two parts separate for safer unmolding, but they have to be really tight to prevent leaks)
OK so there are two types of molds. The left is a one-piece and the second a two-piece mold.
Now you have a base there but it’s upside-down. I’m going to assume that you forgot to flip it over.
Now that is see the image and re-read your question you might be asking how to make the center upside-down piece have the same look as the outside.
In that case you should place the thin piece in the right position above the other. Then make a copy of the second and position the clones on the inner rim of the thin piece and bool them in unity. To make it hollow use the technique I mentioned before.
(you should put a torus to connect the spikes like in the second image)