Make streusel topping by combining the first 1/4 cup measure of butter, brown sugar, 1/3 cup flour and spices in a bowl. Set streusel mixture aside. Quarter each plum and discard pits. Set plums in a bowl and set aside.
Preheat the oven to 350 degrees F. Lightly spray a 10" spring-form cake pan with vegetable oil spray.
To make batter, place 2 cups flour, baking powder and salt in a bowl and whisk to combine. Place the other 1/4 cup measure of butter and granulated sugar in another bowl and beat until well combined. Mix in the egg. Mix the flour mixture and milk alternately into butter mixture. Spoon the batter into the cake pan. Top the cake with the plums, setting them skin-side up and gently pressing them partially into the batter. Disperse streusel mixture on top the cake. Bake for 60 minutes, or until a cake tester inserted into the middle of the cake pulls out clean.