Preheat oven 300F. In med bowl, combine flour, baking powder, and salt with wire whisk. Set aside. Combine sugars in a lg bowl using an electric mixer set at med speed. Add butter and beat until batter is grainy. Add egg, sour cream, and vanilla, and beat at med speed until light and fluffy. Scrape bowl. Add 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 microwave, stirring every 20 sec until melted. Cool chocolate for a few minutes and pour over cookie batter. Using a wooden spoon or rubber spatula, lightly fold melted chocolate into the dough. Do not mix chocolate completely into cookie dough. Drop by rounded T's, 2" apart, onto ungreased cookie sheets. Bake 23-25 mins. Do not brown. Quickly transfer cookie to a cool surface.