Small variation of the blog post https://javatechnicalwealth.com/blog/2018/04/16/cascading-lambdas-and-builder-pattern-in-java-when-1-1-3-but-not-4/ when keeping Person and PersonBuilder in separate classes. The Person class is really just a data class here, but constructor needs to be package scoped instead of being declared private

Person.java

import java.time.LocalDate;
import java.util.Optional;

import static java.util.Objects.requireNonNull;

public final class Person {

    private final String firstName;
    private final String lastName;

    private final LocalDate dateOfBirth;

    Person(String firstName, String lastName, LocalDate dateOfBirth) {
        this.firstName = requireNonNull(firstName, "firstName cannot be null");
        this.lastName = requireNonNull(lastName, "lastName cannot be null");
        this.dateOfBirth = dateOfBirth;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public Optional<LocalDate> getDateOfBirth() {
        return Optional.ofNullable(dateOfBirth);
    }
}

PersonBuilder.java

import java.time.LocalDate;

public class PersonBuilder {

    private final String firstName;
    private final String lastName;

    private LocalDate dateOfBirth;

    private PersonBuilder(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public PersonBuilder dateOfBirth(LocalDate birthday) {
        this.dateOfBirth = birthday;
        return this;
    }

    public Person build() {
        return new Person(firstName, lastName, dateOfBirth);
    }

    @FunctionalInterface
    public interface FirstNameBuilder {
        LastNameBuilder firstName(String firstName);
    }

    @FunctionalInterface
    public interface LastNameBuilder {
        PersonBuilder lastName(String lastName);
    }

    public static FirstNameBuilder builder() {
        return firstName -> lastName -> new PersonBuilder(firstName, lastName);
    }
}