Put the plums on a bowl and sprinkle sugar on them. Leave them to sit while preparing the rest of the ingredients. Pre-heat the oven to 400 degrees. Lightly butter a 9 inch cake tin with a removeable bottom (I used a springform pan) Line with baking paper, butter the paper, and set aside.
Cream together the butter and sugar until light and fluffy. Beat in the egg, lemon zest, vanilla, and sour cream. Whisk the flour, salt and baking powder together in a separate bowl and then add it to the wet ingredients, mixing it in well. Spread this batter in the bottom of the cake tin. (This batter is pretty thick, and the paper will slide around, so I held it down on one edge and spread it out with my fingers.).
Spread the plums over top of the batter in a circular pattern to within 1/2 inch of the edges.
To make the streusel, whisk together the sugar, flour and spices. Cut into the butter until the mixture is crumbly. I use a pastry cutter but a fork will work. Sprinkle the crumbs evenly over top of the plums. Use all the streusel.
Bake on a middle rack for approximately 35-50 minutes until the streusel is golden brown. Remove from the oven and let sit for about 15 minutes on a wire rack, before you remove the sides of the pan and the paper. Serve warm, with whipped cream or ice cream.