Preheat oven to 350℉. Line cookie sheets with parchment paper or non-stick baking mat.
Sift into a large bowl flour, cocoa powder, baking powder, salt and baking soda until well blended. Sift again as needed to fully blend. (Or, using a ballon whisk, whisk until well blended.) Set aside.
In a stand mixer or using a hand mixer and mixing bowl, beat butter on medium speed until light and creamy, about 2 minutes. With mixer on low, add sugar slowly, increase speed to medium and beat for 1 more minute. Add eggs, one at a time, beating after each addition. Continue beating for 1 minute.
Add sour cream and vanilla extra, and mix on low just until combined.
Add flour/cocoa mixture in two additions, beating on low speed ust until blended. Do not over mix. Fold in 1 ½ cups chocolate chips and walnuts if using.
Using a medium ice cream scoop, scoop dough onto prepared cookie sheets, 6 portions per sheet, leaving 2 to 3 inches apart to allow for spread. Bake at 350℉ for 12-14 minutes, until cookies are set. You want them slightly underbaked. In my oven, 14 minutes in the sweet spot.
Remove from oven and allow to cool on the cookie sheet for 5 minutes before transferring to a cooling rack to cool completely.
Melt remaining 1 cup chocolate chips over double boiler and drizzle over cooled cookies. Allow chocolate to set before storing.