Dorie’s recipe by way of Smitten Kitchen with a few notes and expanded to a 9x13 pan.
Ingredients:
8 tablespoons, 1 stick, unsalted butter, at room temperature
215-220 grams, light brown sugar
Grated zest of 1 whole orange and 1 whole lemon
3 large eggs
120 ml flavorless oil, such as canola or safflower
2 1/4 teaspoons pure vanilla extract
3 teaspoons baking powder
1 teaspoon kosher salt
1/2 teaspoon cinnamon or cardamom
290-295 grams all-purpose flour
3-4 peaches and 3-4 purple or red plums, halved and pitted
1 tablespoon turbinado sugar
Center a rack in the oven and preheat the oven to 350°F. Butter an 8-inch square baking pan, dust the inside with flour, and tap out the excess.
Beat the butter at medium speed until it’s soft and creamy, about 3 minutes add the sugar, orange, and lemon zest, and beat for another 3 minutes — I don’t usually worry much about mixing times, but I paid closer attention here and did 3 minutes and 3 minutes here and I recommend it
Then add the eggs, one at a time, and beat for a minute after each egg goes in — Again, if you aren’t on a hurry, I’d recommend taking the full 9–10 minutes Dorie and Deb recommend to mix here
Still working on medium speed, beat in the oil and vanilla; the batter will look smooth and creamy, almost satiny
Sprinkle baking powder, salt, and cinnamon or cardamom over batter and beat well to combine, 20 seconds longer than seems necessary — I don’t know what “20 seconds longer than necessary” is, so I beat it for about another minute total
Add mix flour in with a spatula until just incorporated
Scrape the batter into the pan and smooth the top and arrange the peaches and plums cut side up in the batter — Depending on your plum size, you’ll be able to fit 12 or 15 halves on the top
Bake for about 35 to 45 minutes, or until the top is honey brown and puffed around the plums and a toothpick inserted into the cake between each plum comes out batter-free
Transfer the cake to a rack and cool for 15 minutes during which time the plums juices will seep back into the cake then run a knife around the sides of the pan to unmold the cake and the invert to cool right side up — Because the fruit is so juicy and this cake is so moist, I inverted it first on to a cutting board and then back on to my wire rack and then I left it on the wire rack, loosely tented with foil as we ate over 3 or so days