The cognitive bias that you are looking for is called the Overconfidence Bias. Your description exactly matches its symptoms. With this bias, an expert is more likely to be the victim of overconfidence. This overconfidence, however, is not within the expert's field but within a totally unrelated field in which both the expert and an average layperson have a similar lack of experience. (So, technically, both the expert and a layperson are both laypeople in this field.) For particular reasons, the expert believes that the prowess in their own field will translate over to or aid them with an unrelated field.
A special feature of the Overconfidence Bias is that the expert's overestimation of his abilities increases as the difficulty of the task increases. ie. The more difficult/complex the task is, the more confident the expert is that he can complete the task. Reversely, when the task gets easier, the expert still believes that he can accomplish the task better than an average layperson but his advantage is not as great.
Below is a passage from the Wikipedia page about the Overconfidence Bias which restates the above. https://en.wikipedia.org/wiki/Overconfidence_effect
The overconfidence effect is a well-established bias in which a
person's subjective confidence in his or her judgements is reliably
greater than the objective accuracy of those judgements, especially
when [their] confidence is relatively high. "The Role of Individual
Differences in the Accuracy of Confidence Judgments" The Journal of
General Psychology. 129 (3): 257–299
"The key finding is that confidence exceeds accuracy so long as the
subject is answering hard questions about an unfamiliar topic."
"This phenomenon is most likely to occur on hard tasks, hard items,
when failure is likely or when the individual making the estimate is
not especially skilled."
The above scenario may initially appear to be the Dunning-Kruger Effect because the expert is indeed a novice in an unrelated field and is overestimating his abilities like in the Dunning-Kruger Effect. However, with the Overconfidence Bias, the expert overestimates his abilities more than the average layperson does. Further, the harder the task, the larger the overestimation by the expert. This particular increase in overestimation will not occur in a normal layperson. As you can see, the Overconfidence Bias differs slightly but significantly from the Dunning-Kruger Effect.