Preheat the oven to 350.
If desired, line a 9-inch springform pan with parchment. There's no need to grease the pan.
In a small bowl, combine the flour, baking powder, and salt.
In a mixer or by hand, cream the butter and the 2/3 c Swerve.
Add the eggs one at a time until incorporated.
Add the flour mixture by the spoonful until incorporated.
Drop the thick, sticky batter into the bottom of the pan by the spoonful and spread evenly with a spatula or with your fingers.
Cover the batter with the halved plums, skin-side up.
If desired, squeeze the 1/4 lemon evenly over the top of the plums, careful to remove any wayward seeds.
Sprinkle the 2 tsp Swerve and cinnamon over the top of the torte.
Bake for 45-50 minutes, until the centre tests clean with a toothpick or knife.
Cool for at least 30 minutes before slicing and serving.