Servings: 9 to 16, depending on how you cut it
5 tablespoons (70 grams) unsalted butter, at room temperature
3/4 cup (packed) (145 grams) light brown sugar
2 large eggs
1/3 cup (80ml) flavorless oil, such as canola or safflower
Grated zest of half to 1 whole orange (to taste)
1 1/2 teaspoons pure vanilla extract
2 teaspoons baking powder
1/4 teaspoon kosher salt
1/4 teaspoon ground cardamom or cinnamon
1 1/2 cups (195 grams) all-purpose flour
8 purple or red plums, halved and pitted.
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 and beat for another 3 minutes, then add the eggs, one at a time, and beat for a minute after each egg goes in. Still working on medium speed, beat in the oil, zest and vanilla; the batter will look smooth and creamy, almost satiny. Sprinkle baking powder, salt, and cardamom over batter and beat well to combine, 20 seconds longer than seems necessary. Add flour and mix only until incorporated.
Run a spatula around the bowl and under the batter, just to make sure there are no unimxed spots, then scrape the batter into the pan and smooth the top. Arrange the plums cut side up in the batter — depending on your plum size, you’ll be able to fit 9 or 16 plum halves on the top. A Jiggle the plums a tad just so they settle comfortably into the batter.
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 and unmold the cake. Invert and cool right side up.
Once cool, dust with powdered sugar. You can wrap the cake and keep it at room temperature for up to 3 days, during which time it will get softer and more moist.