In small microwavable bowl, microwave chocolate chips on High in 30-second increments until chocolate can be stirred smooth (microwave about 60 seconds total). Set aside to cool.
Meanwhile, bake 24 cookies as directed on roll; cool completely.
Add cold butter to cooled chocolate in bowl. Beat with electric mixer on high speed until well mixed with no lumps remaining. Add cocoa; beat until fluffy buttercream forms.
Frost bottoms of 12 cooled cookies with buttercream; place another cookie on each, bottom side down, to make sandwich cookies.