Preheat the oven to 350°F (176°C)
Butter a 9-inch springform pan
In a large bowl, combine the flour, baking powder, and salt
In a large bowl with a stand mixer or hand mixer, beat the butter and 1 cup sugar until light in color, 3 to 5 minutes
Add the dry ingredients
Add the eggs, 1 at a time, mixing just until combined
Spoon the batter into the pan
Cover the top of the batter with the plum halves or wedges, skin side up
Sprinkle with the remaining tablespoon of sugar and the Somerset Ridge Citron, adjusting the amount to the tartness of the fruit
Sprinkle with the cinnamon
Bake the plum torte until the cake is golden and the plums are bubbly, 50 to 60 minutes
Cool on a rack, then unmold