1. Preheat the oven to 375F. Grease a 9" round springform pan and set aside.

Make the Streusel

1. In a medium sized bowl, mix the flour, cinnamon and salt together. Add the sugars and mix until distributed. Add the butter and mix with your fingers until the streusel start to form large clumps. Place the bowl of streusel in the fridge to chill.

Make the Cake

1. In a small bowl mix together the flour, baking powder, baking soda and salt. Set aside. In the bowl of a stand mixer with the beater attachment or in a large bowl with a hand held mixer, beat the butter and sugar until light and fluffy, about 2 minutes. Add the eggs and beat again to combine. Add the vanilla extract and sour cream and beat again until combined. Add the dry ingredients and mix on low until fully combined.

2. Drop dollops of the dough all over the prepared springform pan, then spread evenly with a spatula. Sprinkle the dough with bread crumbs or ground almonds.

3. Wash, slice each plum in half to remove the pit, then cut each half again so each plum is cut into quarters. Arrange the plum quarters in concentric circles on top of the bread crumbs or almonds, placing the plums as close together as possible. Sprinkle the plums with cinnamon if desired. Remove the streusel from the fridge and re-crumble it with your hands as you sprinkle it evenly over the plums.

4. Bake for 30-35 minutes or until golden brown on the edges and bottom, turning the cake halfway through the baking time to ensure it bakes evenly. Remove and allow to cool for 10 minutes before sliding a small sharp knife around the edge of the pan to loosen the cake, then remove the springform and allow to cool completely before removing the cake from the bottom of the pan.

5. Best served with a gigantic spoonful of lightly sweetened whipped cream!