I want to spend a good chunk of time baking something delicious and slightly ambitious... what do you make when you want to bake a day away?

Macarons! When we were snowed/iced in over the last week I made 5 batches trying to perfect my method. My freezer is filled with yummy cookies. I really didn't need cookies after the holidays but when you're stuck inside...

Cookies were hardly the first thing I thought of when I saw this question, but I recently passed a good hunk of a cold day making Ischler cookies. I used Maida Heatter's dough recipe- she does it by hand, starting by blanching a couple of cups of almonds- the dough is similar to a Linzer dough (note- Heatter says to roll to 1'4", which is too thick). For filling I made a Lekhvar from a recipe (Polish Christmas Cookies, from this site)- after making the cookies and putting them together (it's a sandwich cookie) and half the sandwich is dipped in chocolate- all of this takes quite a bit of time (and makes a considerable mess). Then, you should have enough of the Lekhvar left over to make a load of Rugelach in the afternoon (both of these doughs need refrigeration). Rose Levy Beranbaum (who needs a shorter name- Rosie B.?) also has aan Ischler cookie recipe that's pretty good- and a bit different- but I like this one.

It's not as fancy as some of these other suggestions, but I love to make challah when I have a little bit of time. I use the recipe in Smitten Kitchen's cookbook (https://smittenkitchen.com/2012/09/fig-olive-oil-and-sea-salt-challah-book-tour/) and it is so beautiful when it comes out of the oven. I would love to tackle croissants one day but need to have somewhere to bring them :)

Babka! Bagels! Homemade puff pastry!

Happy early birthday! Here are a few ideas from around the site:
https://food52.com/blog/15801-20-weekend-baking-projects-to-conquer-this-winter-and-spring
https://food52.com/blog/8309-homemade-croissants
https://food52.com/recipes/51577-pizza-swirl-bread
https://food52.com/recipes/56541-s-mores-layer-cake
https://food52.com/recipes/39542-brown-butter-butternut-rolls
https://food52.com/recipes/40319-triple-layer-mousse

Anything laminated. If I want to get my hands into something, put a good bit of elbow grease into it and get something really magical afterwards, I make croissants or puff pastry or danish dough....and if its not yeasted, I make a bunch of it. A few times a year I'll just make a few pads of puff and keep it frozen. SO versatile and so worth it. With it you can make mille feuille, fruit danish/tarts, pot pies, hand pies, turnovers, kouign amann, cheesey turney straw things, etc. That's my zen.

Since its your birthday, you could build on the puff a little more and make St. Honore which brings together a few technique-driven applications like the puff, pate a choux, caramel, creme chiboust, etc. Get the piping tips out, make spun sugar or ganache or praline and just bake and decorate your little heart out. Its fun, involved and really makes an impact.

I have been wanting to make Pattiserie-type mousse cake, the ones that has many different layers annd filling, and so artistically decorated, like http://freshnewengland.com/fresh/2009/11/elegant-cranberry-and-white-chocolate-gateau.html , http://seelenschmeichelei.de/espresso-mousse-cake-mit-mirror-glaze/ , https://s-media-cache-ak0.pinimg.com/originals/94/d4/21/94d42124abc07e6c7c35df153c18b7b3.jpg , http://the-wandering-girl.blogspot.fr/2012/03/entremet-au-matcha-et-sesame-noir.html , http://antoinette.com.sg/products-wholecake.html , http://www.prettytastycakes.com/2008/05/28/opera-cake-with-apricot-mousse-and-pistachio-buttercream/ , http://chloedelice.blogspot.fr/2014/08/entremets-mangue-passion-insert.html .

Dobos Torta, a classic Hungarian cake and first choice for birthday cake in our family. It's 7 layers of sponge cake layered with a chocolate custard buttercream, and optionally topped with a thin layer of caramelized sugar. And it freezes well!
I would also be tempted to make any kind of Austro-Hungarian Torte that catches my attention. Birthdays are all about the cake!
Let us know what you decide to do...

Chocolate Babka! Takes some time but it is SO worth it😋

Panettone recipe by Jim Lahey

Croissants or bagels? Food52 has good articles and recipes for both of them. Or if you want to go the birthday cake route, I, after encouragement from a Hotline question, had my first adventure with rolled fondant this year. It was definitely a special project. Happy birthday!

Paris-Brest, a real showstopper and a bit of a stretch for many experienced home bakers. I use Alice Medrich's rice flour pastry cream and for the rest, the recipe from Cook's Illustrated, substituting almonds for hazelnuts because we just like them better. I seriously raised the bar for Christmas desserts, making one a few years ago.
If you need the Cook's Illustrated recipe, please send me a private message via my profile. I cannot post it due to copyright ownership, but I can share it individually, as that is "fair use."
Here are more details, though not the recipe I use: http://www.joyofbaking.com/ParisBrest.html
Cheers, and happy baking, and birthday! And what a lovely gift to yourself.
AJ ;o)

kouigns amann. i've made the recipe from the Baking Bible by Rose Levy Beranbaum, and it is fantastic. Yosi Arefi posted a recipe here: https://food52.com/recipes/27245-kouign-amann

Brioche (any good recipe, but I like Julia Child's)...which, of course, can be made plain or fancy, filled, oe served with luscious jams and coffee.
Schwarzvalder kirschtorte...aka
Black Forest Cake (chocolater sponge layer cake with 2 kinds of cherries and 2 flavors of whipped cream. Oh, and did I mention the chocolate or the eau-de-vie? My go-to recipe is from CIA (not Langley; the one on the Hudson), I believe by Chef Albert Kumin.