German Plum Cake

I used to be really into baking before I got all crazy healthy and what not. But I slowly started trying out new recipes and testing them with my family.
I received a ton of fresh plums from a friend and knew I couldn't eat all of them so I decided to make a German Streusel plum cake.
The Cake
120g butter
1/3 cup sugar
1 tsp vanilla
1 large egg
1 tblspn sour cream
1 pinch salt
grated zest of one lemon
1 3/4 cups plain flour
1 tsp baking powder
2 cups plums, pitted and sliced into wedges
For the Streusel
85gm butter
3/4 cup plain flour
1/3 cup sugar
1 tsp cinnamon
1/2 cup of quick oats
Pre-heat the oven to 350F
Line a 9-10” removable bottom cake pan with parchment and grease the sides.
The Cake
Cream together the butter and sugar until light.
Beat in the egg, lemon zest, vanilla, sour cream and salt. Sift in the flour, baking powder. The batter is very think so just pour it into the pan and even it out with a spoon.
Spread the plums evenly on top of the cake batter.
The Struesel
To make the streusel topping place all the ingredients in a bowl with your butter at room temp.
Cut the butter into the dry ingredients until it is clumping together well. Sprinkle it over the top of your plum layer.
Bake in the pre-heated oven on the middle rack for approximately 45 minutes to an hour, until the topping is nicely browned and the cake tests done. Remove from the oven and let sit for about 15 minutes, cooling on a wire rack, before removing the sides of the pan.
Serve warm with a dallop of cool-whip or vanilla ice cream