Preheat oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large bowl, beat softened butter and granulated sugar until light and fluffy.
Beat in eggs one at a time, mixing well after each. Stir in vanilla extract.
Gradually add dry ingredients to wet, alternating with milk. Mix just until combined.
Pour batter into prepared pan and spread evenly. Arrange sliced plums on top.
To make streusel, combine flour, brown sugar, and cinnamon in a small bowl.
Cut in cold butter with fingers or pastry blender until mixture resembles coarse crumbs.
Sprinkle streusel evenly over plums.
Bake 45 to 55 minutes until a toothpick inserted in center comes out clean.
Cool cake in pan 15 minutes, then invert onto wire rack to cool completely.