How can I let a plugin.dll call functions of core-application?

okay. So I want to build up a program, that acts as a core for "plugin"-modules. Another developer could create a plugin1.dll and add it to the "modules" folder to enhance the functionality of my core-application.

So lets say my core has as example those functionalities:
- Logging
- User Authentification
- User Interface

As example we have our core-application as mentioned above and someone wants to add a plugin that lets an user see the current time and log it into a standard-log.txt.

So he would create a class-library that has the functionality: - get the current time (functionality included in the .dll) - display the current time (functionality included in the .dll) - log the current time (functionality included in the core)

Now my problem is, I can invoke the functionalities of the plugin easily from my core-application using reflection, but how would I the other way around?
How can my plugin1.dll access and invoke the fully-set up logging-functionality of the core-program?

I hope you got my question. I want my plugin1.dll to be able to call as example Logging-methods of my core-class.

Thanks!