It starred Sōta Aoyama as Matsuda, reprising his role from the other live-action movies in the trilogy (soon to be quartet).
As with the main movie L Change the World, Kiyomi Fujii wrote the screenplay. The spin-off covers Matsuda's issues with filing a final Kira case report, and a poignant meeting with L a fortnight before the latter's known and inevitable death.
Spin-Off Matsuda: L Change the World Special and Synopsis
The most common copy of Spin-Off Matsuda available on the internet is embedded here - in Japanese with Thai sub-titles.
There are no versions with English sub-titles on-line (or may not have even been made), but a brief synopsis runs like this:
Touta Matsuda's signature is required upon a document concluding the Kira case. He waivers over signing, because he's not sure that the case should be closed. He can't get his head around what precisely Light believed with regard to justice, nor what L did to thwart him.
Soichiro Yagami pre-empted this and left a note for Matsuda stating, 'Go see Ryuzaki'. So Matsuda goes to visit L, carrying with him a gift - a box of cakes.
L gratefully receives them with the comment, "A farewell gift to a person going for death. I shall thank you."
This incenses Matsuda, who can't understand why L continues to work, when he only has two weeks left to live. "Stop working!" He rants, "Do anything you like, such as dating with a girl, or travelling anywhere you like, whatever. Why don't you spend the rest of your time for yourself! At least do not pretend dying is no big deal!"
Once he's calmed down, Matsuda is contrite about his behaviour. He calls Soichiro and apologizes for being mean to Ryuzaki.
Just on it, a delivery man brings a gift from L - a basket of crepes with the message, 'Sugar will improve your brain function. Eat lots of crepes to become clever - YOU IDIOT!'
Matsuda exclaims, "L, you Hyottoko guy!" (Hyottoko being the mask worn by L in the movies.) And stands there eating, crying and pondering on how L can bear knowing that he only has 23 days left to live, yet still carry on.
The movie short ends with Matsuda crouched in the street, overwhelmed by the sentiment and his emotion, dwarfed by the architecture of the city around him.