Take a sphere, with its equation:
x^2 + y^2 + z^2 <= r^2
Evaluate for each block, if it's true, set block to endstone, otherwise set to air.
Now you want to distort the sphere a bit, so add some 3D noise:
(x + NoiseX)^2 + (y + NoiseY)^2 + (z + NoiseZ)^2 <= r^2
You should get a "clump" shape. Done.
For optimization, you can add some initial tests - if the chunk is too far away from the [0, 0], you can be certain that there's nothing to be generated there, so just fill with air without evaluating anything.
Would you like to try writing that?
I would recommend writing it in C++ as a separate CompositionGen. The cNoise class has no Lua API, so you'd need to implement your own noise functions, and it would be super-slow in Lua.