Sift togeather the flour, baking powder, baking soda, salt, and cocoa powder into a bowl. Whisk the mixture and set it aside.
Melt the semisweet and unsweetened chocolate in a double boiler or in a bowl placed over a pot of simmering water. Cool to room temperature and set aside.
In the bowl of an electric mixer using the paddle attachment, cream the butter and sugars togeather on medium speed for 1 to 2 minites, or until the mixture becomes somewhat light.
On low speed, add the eggs, one at a time, incorporating each one well before adding the next. Stop the machine occasionally and scrape down the sides of the bowl.
With the machine still on low speed, slowly add the cooled melted chocolate and blend well. Stop the mixer and scrape down the sides of the bowl. Add the sour cream and vanilla extract, blending well.
On low speed, add the flour and cocoa mixture and blend only until combined. Remove the bowl from the electric mixer and, using a rubber spatula, blend in the chocolate chips by hand.
Using a 1 ounce ice cream or cookie scoop, place scoopfuls of batter onto a sheet pan covered with parchment paper; pace each scoop 3 inches apart. With a moistened hand, flatten each cookie slightly. Bake for about 18 minites. The fudge cookies with be soft, but will firm up when they cool. Allow the cookies to cool on the sheetpan for 5 minutes before removing and placeing them on cooling racks.