I got this recipe from a friend, and the frosting was great but the cookie part came out really dry and hard when I made it. So I'll include that original cookie recipe at the bottom (in case you like hard/dry oreos), but the main recipe here is adapted from a different recipe I had before.
Makes about 32-36 cookies (so 16-18 finished oreos)
3/4 c butter
1 c + 2 Tbs sugar
2 regular eggs (only 1 if extra large)
1 t vanilla
2 c flour
1/2 c cocoa powder (add with sugar or flour)
Whip butter, add sugar gradually, whipping frequently. Add eggs and vanilla and mix well. Add flour and cocoa and mix well. It will probably seem a bit dry, but keep mixing and it should moisten up. Dough will probably be a bit sticky. You can chill it, but it will probably be firm enough to just roll out right away. Use plenty of flour (I use a pastry brush to brush off excess flour so the cookies stay brown rather than white). Roll 1/4 inch thick and cut into 2 inch rounds. Bake at 350 for 10ish minutes.
1/2 c butter, softened
1/2 tsp vanilla
1 1/4 c powdered sugar
Whip together until light and very fluffy. Pipe onto one cookie and put another on top.
Alternately, use a little cream cheese in place of (or in addition to) the butter. The batch in the photo I made part plain frosting, then added some strawberry cream cheese to the other half and made strawberry oreos, which is why the ones on the left are pink. They were fantastic. ☺
Original chocolate cookies:
1/2 c soft butter
1/2 c confectioners sugar
1/2 c cocoa powder (dark is best)
1/2 tsp salt
1/2 tsp vanilla
2/3 c flour
Cream butter until fluffy, add the sugar and mix well. Add the cocoa and vanilla and mix well.
Work in the flour until dough is soft but not too sticky. Knead lightly and chill for 40 minutes.
Dust chilled dough lightly with flour, then roll out 1/4 inch thick between two pieces of wax paper. Cut into 2 inch rounds. Bake at 350 for 10-12 min. Cool.