Melt the butter in a large microwave-safe mixing bowl. Add chocolate to hot butter and stir well. Microwave for another 30 seconds and stir until chocolate is melted and smooth. Let mixture cool for about 5 minutes.
Beat the sugar into the melted chocolate mixture. With an electric mixer on medium, beat in the eggs one by one. Add the vanilla, increase mixer speed to high and beat for about a minute and a half. Batter will fluff up and appear lighter in color.
Mix the flour, cocoa powder, baking powder and salt together in a separate bowl. By hand, stir it into the batter until incorporated. Stir in about 3/4 cup of the chocolate chips (you can use less if you like). Chill the batter for an hour or more or until it is firm enough to shape into fairly sturdy mounds.
Preheat oven to 350 degrees F. Line sheets with parchment paper. Scoop up big two-tablespoon sized mounds and arrange them on cookie sheets a few inches apart. Bake one sheet at a time for 8-10 minutes. Cookies should still be a little shiny when you take them out, but baked. Mine took 10 minutes.
Let cool on cookie sheets for a few minutes then transfer to cooling racks.
To make icing, melt the chocolate chips and butter together in microwave or in a metal bowl set over hot water. Drizzle the chocolate over the cookies or if it’s too thick to drizzle, transfer it to a freezer bag, snip off the corner and squeeze it over the cookies.
When made with Dutch process cocoa, my cookies turned out thicker.