Preheat oven to 300°.

In medium bowl, combine flour, baking powder and salt with wire whisk. Set aside.

Combine sugars in a large bowl using an electric mixer set at medium speed. Add butter and beat until batter is grainy. Add egg, sour cream and vanilla, and beat at medium speed until light and fluffy. Scrape bowl. Add the flour mixture, and blend at low speed until just combined. Do not overmix.

Place chocolate chips in double boiler over hot but not boiling water. Stir constantly until melted. Or, place chips in a microwave-proof bowl and microwave on high, stirring every 20 seconds until melted.

Cool chocolate for a few minutes and pour over cookie batter. Using a wooden spoon or rubber spatula, lightly fold melted choclate into the dough. Do not mix chocolate completely into cookie dough to get the marbled effect.

Drop by rounded tablespoons, 2 inches apart, onto ungreased cookie sheets. Bake 23-25 minutes. Do not brown. Quickly transfer cookies to a cool surface.