1 1/2 cups all-purpose flour
2 teaspoon baking powder
1/4 teaspoon salt
Scant 1/2 teaspoon ground cardamon (optional)
5 tablespoons unsalted butter, at room temperature
3/4 cup (packed) light brown sugar
2 large eggs
1/3 cup flavoeless oil, such as canola or safflower
Grated zest of 1 lemon
1 1/2 teaspoons pure vanilla extract
8 purple or red plums ( in the fall, use Italian prune plums), halved and pitted
Center a rack in the oven and preheat the oven to 350 degrees F. Butter an 8-inch square baking pan, dust the inside with flour, tap out the excess and put the pan on a baking sheet.
Whisk the flour, baking powder, salt and cardamom, If you are using it, together.
Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand in a large bowl, beat the butter at medium speed until soft and creamy, about 3 minutes. Add the sugar and beat for another 2 minutes. Add the eggs one at a time, beating for a minute after each addition. On medium speed, beat in the oil, lemon zest and vanilla. The batter will look very light and smooth, almost satiny. Reduce the mixer speed to low and add the dry ingredients, mixing only until they are incorporated.