This was a tough one...after all, it's Friday. But when all was said and done, I'd have to say that it may be the most delicious themeless puzzle I've ever seen.
The long fill, both top and bottom, are the best.
1A: Oblong dessert (banana split). We had this earlier in the week, but it's not where I was going with the clue. I was thinking something baked...maybe something chocolate.
12A: Compositions (musical pieces). Again, I was off base, heading in the direction of the written word.
14A: Wizards and Magic, e.g. (basketball teams). I can't tell you how excited I was to figure that one out with only a few letters in place.
45A: Hiding one's true feelings (putting up a front).
47A: French-born architect who designed Washington, D.C. (Pierre L'Enfant). I think we've had him in a puzzle before.
48A: 1930 novel that takes its title from Shakespeare's "Twelfth Night". (Cakes and Ale). I didn't have a clue. Ah...Somerset Maugham.
A kind-of-Scrabbly vertical answer spans the grid at 6D: It separates the Bering Sea from the Pacific (Alaska Peninsula). I figured out pretty quickly that it was a peninsula, but it took a while to get the first word. It crosses at the E with 27A: Old masters reside in them (galleries)...one of only a few gimmes for me.
Other gimmes include:
23D: Wilbur Post's "pal" (Mr. Ed). That made its crossing at 23A: Drab and colorless (mousy) easy to get.
33A: Biblical figure who says to God "Make me understand how I have erred" (Job). That's one you could guess even if you didn't know he'd said that. I mean, no one else was tested the way Job was. That put me on the right path for its cross at 33D: Paleontologist's discovery, maybe (jawbone).
42A: Doesn't stay on topic (meanders). This was actually a good guess that panned out.
44A: Gelato sans milk (sorbet). Mmmmmmm. A nice treat for those of us who can't do dairy. And much more appetizing than 27D: Ice cream flavor (green tea). I like to drink green tea, but I can't imagine it's tasty as ice cream.
Some clever cluing to add to the interest:
21A: Skull Island denizen, for short (Kong).
41A: Long known for playing football (Howie). Took a good minute to realize they wanted a first name.
3D: Potential heiress (niece). When I had the final E in place, I wrote in niece, then erased it. Once I figured out attics (16A: "Heat traps" in houses), I went back to it. I thought they wanted some kind of royal heiress...although one of my nieces thinks she's a Princess. (If you're reading this, Maren...Auntie loves you!)
29D: Early "astronaut" (chimp). I think we've been asked for the name of the first one...that would be Enos.
38D: Stock holder (corral). Not the investment kind of stock.
40D: Romance or horror (genre).
Nice to see the talented Aretha at 26A: "__ Now" (1968 R & B album). Also liked 31D: Undependable (erratic) and 34D: Show the ropes to (orient).
Here's the grid...hopefully it's error free.
See ya! (15D: "G'bye!").