Grease an 8 inch round spring form pan, line with parchment paper then grease the parchment paper. Set aside.
Preheat the oven to 350 degrees F.
In a large bowl beat together the softened butter with the brown sugar until fluffy.
Beat in the eggs one at a time just until combined.
Stir in both the vanilla and almond extracts.
In a medium sized bowl, whisk together the flour, finely ground almonds, baking powder, baking soda, cinnamon and salt.
Stir the flour mixture into the butter mixture alternating with the sour cream, making 3 additions of the flour and 2 of the sour cream, just until smooth.
Scoop the batter into the prepared cake pan and smooth the top with the back of a spoon.
Slice the plum halves length-wise into 1/4 inch slices and while keeping the slices together place them in clusters over the cake batter.
Sprinkle the streusel topping between and around the sliced plums and top with the sliced almonds.
Bake in the preheated oven for about 40 to 45 minutes or until a cake tester inserted in the middle comes out clean.
Allow the cake to cool in the pan for about 20 minutes before releasing it from the pan.
Once completely cooled dust the cake with powdered sugar, slice and enjoy!