Preheat oven to 175 degrees Celsius. Lightly butter a muffin tin.
In a medium mixing bowl, whisk together the flour, baking powder, cinnamon and salt. Set aside.
In a mixing bowl, cream butter for about 2 minutes on medium speed. Add in beaten egg, sugar and vanilla and beat on medium-high speed until mixture is pale and fluffy, about 3 to 4 minutes, scraping the sides occasionally. With the mixture running on low speed, gradually add in all the flour mixture, mixing until just combined (don't overmix!) Use a spatula and scrape the bottom of the bowl, ensuring all the flour mixture is incorporated.
Divide the dough evenly among the muffin tins. I ended up making 8 tartlets, so adjust according how high you want your tartlet to be. Arrange the plum slices on top of the tart, gently pressing the slices into the dough. Sprinkle the top with muscovado sugar.
Bake for 25 to 30 minutes, until the top has caramelized to a golden brown and the tartlets look set. They are yummy when warm, at room temperature or chilled!