Preheat oven to 350°F. Butter and flour a 9-inch springform pan. Optional - if macerating the raisins, soak them in rum for at least 20 minutes, pressing down on the raisins occasionally to make sure the top ones get soaked, too.
With a mixer beat the butter and brown sugar on medium-high until light and fluffy. Beat in the eggs, one at a time. Stir in the vanilla extract, pumpkin puree, and sour cream. Drain the raisins from the rum (if macerating) and stir in.
Stir in the flour mixture until just combined. Spread the batter in the prepared pan and then sprinkle the topping over the batter.
Bake for about 1 hour or until a toothpick comes out clean when inserted into the middle of the cake. Allow to cool, then remove from pan and dust with confectioners sugar. Cut and serve.