Which of the major authors or codifiers of halacha. . .
a) refer directly to or quote from kabala in their legal discussions?
b) employ conclusions based only on kabala in their legal argumentation and decisions?
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First as far as being posek al pi Kabbalah the inyan is only L'Humra not L'kula. For instance the Zohar says one needs only to wait a half hour between eating meat and eating milk, but we do not hold that way.
Aside from that the list becomes quite extensive as Chanoch also said, from the Ramban down to today amongst Sephardi poskim. Of the modern Sephardi poskim, there is Rav Mordekhai Eliyahu ZTz"L, Rav Ovadiah Hedayya ZTz"L, Rav Ezra Attiah ZTz"L, Rav Yehuda Tzedaka ZTz"L, Rav Ben Tzion Abba Shaul ZTz"L, Rav Ovadiah Yosef ZTz"L, Rav Shlomo Amar Shlita ect. ect...
Then there were the Misnagdim, such as the Magen Avraham, GR"A and R' Haim of Volozhen.
Then there are the Hasidic Admurim.
Essentially poskim who consider what is written in Kabbalistic sefarim can be found in nearly every stream of Orthodox Judaism.
As far as employing conclusions based solely on Kabblah, not even the Ar"i HaKodesh did that. A good example is the time of laying Tefillin, al pi Kabbalah any time after Hatzot HaLailah is suitable. However the AR"I in defference to the Gemarra and prior poskim, restricted the time to no earlier than Alot HaShahar. There are excellent work ups on the entire Teshuva in R' Ovadiah Yosef's Halichot Olam, and R' Apjin's sefer Divrei Shalom, unfortunately neither are online. Ultimately the essential point is that the Kabbalah, is the inner aspect of the Torah, as such, and as many Mekubalim have elucidated to the point where to list them all would be an exercise in futility, though it is expoused briefly in the introduction to Eitz Haim, and the introduction of Sha'ar HaKavvanot, the Kabbalah gives a deeper meaning to the mitvot, and halakha, but can never directly contradict the p'shat(aka Halakha).
The Alter Rebbe writes in his Shulchan Aruch HaRav Orach Chayim 25:28 (quoting the Magen Avraham 25:20): That when Kabbalah and Halacha conflict we only hold al pi Kabbalah when it is more stringent. In addition his S.A. has a great deal of Kabbalistic sources which he uses to come to a halachic conclusion.
An example of this is discussed here in regards the Baal haTanya's donning shel yad seated.
It's a very important part of Sephardic psak. Among the authorities of interest to look at: The Gaon HIDA, R' Chaim Palagi, R' Yaakov Hayyim Sofer (Kaf HaHayyim), R' Yosef Hayyim (the Ben Ish Hai), and to a lesser extent R' Ovadia Yosef.
R' Shneur Zalman of Liadi and other Chassidic poskim also poskin based on on kabbalah a lot.
Rav Yaakov Moshe Hilel Shlita wrote an entire BOOK on this- called Geburat HaAri, where he talks about kabala and halacha mixing. Not quoting from his book, but basically the general approach is that if there is a Machloket Gemara-Zohar (Arizal) then we follow Gemara. However, if there is a Zohar (Arizal) in a case where there is NO Gemara, then we follow the Arizal. That's the generally accepted approach, but I recommend that book because he brings down the all the Rishonim and Aharonim's opinions and explains and goes it to much more detail. [He also discusses this in his book- Ed HaGal HaZeh]
The source to wind the hand tefillin 7 times around the arm is based on Kabbalah (Niglah doesn't mention any amount of times one winds it around (See here and here)). Yet, the custom is to tie it 7 times (even though one isn't allowed to interrupt between the tfillin shel yad to shel rosh at all.)
Two sources: The Vilna Gaon on Shulchan Aruch Yoreh De'ah Siman 89 Seif Koton 11. Kabbalah can be a reason to be Machmir. The Mishna Brura Siman 25 Seif Koton 42: Same as a Vilna Gaon. Also, if something is only mentioned in Kabbalah and not in the Poskim, we should act that way but "ein lakof" - we don't compel people to do it. If there is a Machlokess Poskim and the Kabbalah is like one side we follow the Kabbalah's side.
Reuvein Margolies wrote a book (article?) called HaRambam V'Hazohar, which apparently (I've never read it) shows places where the Rambam quotes the Zohar (translated into Hebrew) in the Mishnah Torah. If anyone has a link to where I could read this sefer online, that would be great.
Also, the Rambam in Hilchos Mezuzah (5:4), says that "Minhag Pashut" to write Sha-ddai on the outside of the Mezuzah, corresponding to the blank space between the Parshiot in the Mezuzah. The Kesef Misha (R' Yosef Cairo, who was a Kabbalist), quotes the Zohar that says we write it corresponding to the word "V'Haya". (I'm not sure what we do in practice, but if we did as the Kesef Mishna says, that would be paskening according to the Zohar).
Also, what about the Dutch custom of waiting an hour between Meat and Milk. When it talks about it in Shulchan Aruch, the GR"A quotes the Zohar. See the discussion thread on this answer.
This Sefer was just uploaded to Hebrewbooks.org. It is called Sefer Zohar HaRambam (published 5773 in Jerusalem) and says it has 22 concepts from the writings of the Rambam that appear to be based on Kabbalah and the Zohar. I have not read the sefer.