Preheat the oven to 300 degrees Fahrenheit (160 degrees Celsius). In a medium bowl and using a wire whisk, combine the flour, baking powder, and salt. Set aside.

Combine the brown and white sugar in the bowl of the mixer. Add the butter and beat at medium speed until the batter is grainy. Add the eggs, sour cream, and vanilla and beat at medium speed until the batter is light and fluffy. Scrape the bowl. Add the flour mixture and mix it in at low speed until it is just combined. Don’t over-mix.

Place the chocolate chips in a double boiler over hot but not boiling water. Stir constantly until the chocolate is melted. Alternately, place the chips in a microwave-proof bowl and microwave on high, stirring every 20 seconds until the chocolate is melted. Cool the chocolate for a few minutes and pour it over the cookie batter. Using a wooden spoon or rubber spatula, lightly fold the melted chocolate into the dough, stirring only until the chocolate is well distributed and creates a swirled, marbled pattern. Do not mix the chocolate completely into the cookie dough!

Drop by rounded tablespoons, two inches (five centimeters) apart, onto ungreased cookie sheets (I line mine with Gefen Parchment Paper). Bake approximately 20 minutes. Do not brown. Quickly transfer the cookies to a cool surface. Drizzle with melted baking chocolate, if desired.