Heat oven to 350 degrees. Butter 9" round cake pan.
Place 1-2/3 cups flour, the granulated sugar, baking powder, 1/3 cup butter, and the eggs in mixing bowl. Stir with wooden spoon until blended. Transfer to prepared cake pan. Flour fingers and pat dough over bottom and up sides of pan. Fill dough-lined pan with plums.
Stir remaining 1 cup flour, the brown sugar, cinnamon, and nutmeg together in mixing bowl. Add 1/2 cup cold butter and rub mixture between fingers until thoroughly blended. Sprinkle crumb topping evenly over plums.
Bake cake until golden brown, about 40 minutes. Let cool completely in pan on wire rack.
To serve, run knife around edge of pan. Invert cake onto plate, then quickly invert again onto serving plate. Cut into wedges and serve (with vanilla ice cream, if desired).