How do I subclass the build command?

The subject is self-descriptive: I need to subclass the setup.py build command in order to perform additional build steps. However I've failed to find any build command class to inherit from. I've been trying:

class BuildCommandProxy(setuptools.command.build):
    pass

and

class BuildCommandProxy(distutils.command.build):
    pass

and even:

class BuildCommandProxy(setuptools.distutils.command.build):
    pass

without any success.

UPDATE: looking for how to implement something like this with setuptools.

UPDATE2: I have some custom command implementation:

class CustomCommand(setuptools.Command):
    # ...

What I would like to implement is to pass this command to cmdclass like this:

cmdclass={
    "build": CustomCommand,
}

and then invoke the original build in CustomCommand.run after doing some custom steps.