Decode base64 encoded JSON to POJO with jackson and spring-boot

I have a request coming like this

{
    "varA"  : "A",
    "varB" : "TCFNhbiBKb3NlMRgwFgYDVQQK"
}

where key varB is a base64 encoded JSON string. something like this:

{
    "nestedVarB1": "some value here",
    "nestedVarB2" : "some other value here"
}

I want to convert it into some POJOs like this:

@Data
public class MyRequest {
    private String varA;
    private B varB;
}

@Data
public class B {
    private String nestedVarB1;
    private String nestedVarB2;
}

I went through several solutions on stack overflow like this and this, but I want to convert the JSON directly into an object of type MyRequest maybe by writing some sort of Jackson deserializer.

How can I convert the JSON directly into MyRequest using Jackson and Spring Boot?

Note:

1. POJOs MyRequest and B are very big and B could be further nested, so doing it manually would be a big task.
2. I don't have any control over the request coming, it is coming from a third party source. So, can't change the format of the request.