Preheat the oven to 350F. If you are making a cake, grease and line a 9 inch springform pan. If you are making muffins, grease or line approximately 12 standard muffin tins.
In a small bowl, whisk together the flour, baking powder and salt.
In a mixer fitted with the paddle attachment, cream the butter and sugar until fluffy and light. Add the eggs one at a time, making sure they are fully mixed in to the batter. Add the dry ingredients and mix until just combined.
Spoon the batter into the muffins cups or cake tin and smooth the top. Arrange the plums, cut side down, all over the top. You may need more or less plums, go on and crowd them on there.
Sprinkle the lemon juice over the plums. Then sprinkle the cinnamon and sugar. (I'd mix the cinnamon and sugar together and then sprinkle.)
Bake until cake is golden. A cake will take about 45 minutes. Start checking the muffins at about 25 minutes.
These little beauties are even better the next day when the plum juices have had time to settle into the batter.