As with all cases you ever do, the most important thing to remember is structure. I would set context in a few sentences then dive into the challenge. Finishing off with a recommendation or summary.
Consultants also love bullets - it simplifies issues and the simpler you make the problem, the better your answer is.
Written case interviews are indeed becoming very fashionable nowadays as a way to interview! Remember that the skillset tested is the same than in the "usual" cases, hence, all the practice you may have done totally plays in your favor.
The executive summary is usually placed at the beggining, the 1st slide out of 4-5 -depending on prep time-. It´s nothing further than the messages -action titles,indeed- from the slides that are going to follow.
One important point to add is the need to be very 80-20, structured and to the point, since the prep time is very short, so we need discipline with the analysis to have enaugh time to prep the communication strategy.
There are many many entries in thsi same Q&A regarding written cases, hence, I would recommend you to look with the keywords "written case"
Hope it helps!