Create a new backend controller 

If you do not have one yet, create a basic extension to put the controller in.

vendor/bin/typo3 make:backendcontroller

typo3/sysext/core/bin/typo3 make:backendcontroller

You will be prompted with a list of installed extensions. If your newly created extension is missing, check if you installed it properly.

When prompted, choose a name and path for the backend controller. The following files will be generated, new or changed files marked with a star (*):

$ tree src/extensions
└── my-test
    ├── Classes
    |   └── Backend (*)
    |   |   └── Controller (*)
    |   |   |   └── MyBackendController.php (*)
    ├── Configuration
    |   ├── Backend (*)
    |   |   └── Routes.php (*)
    |   └── Services.yaml (*)
    ├── composer.json
    └── ext_emconf.php

Learn how to turn the backend controller into a full-fledged backend module in the chapter Backend modules API.