dimply plum cake

INGREDIENTS

• 1 1/2 cups all-purpose flour
• 2 teaspoons baking powder
• 1/4 teaspoon salt
• Scant 1/4 teaspoon ground cinnamon
• 5 tablespoons unsalted butter, at room temperature
• 3/4 cup (packed) light brown sugar
• 2 large eggs
• 1/3 cup flavorless oil, such as canola or safflower
• Grated zest of 1 orange
• 1 1/2 teaspoons pure vanilla extract
• 8 purple or red plums (or even Italian prune plums, when they are in season), halved and pitted