Preheat oven to 400 degrees.
In medium bowl, sift together the flour, cocoa powder, baking soda, and salt.
In stand mixer, beat together butter and sugar until light and fluffy. Add in egg and vanilla.
Slowly mix in dry ingredients all together and mix until combined.
Using a scoop, place 1 1/2 inch size dough balls on baking sheet (about 12 per sheet). Bake for about 8 minutes. Once baked, cool completely.
Meanwhile, in a medium bowl, mix together butter, brown sugar, and white sugar. Add in milk and vanilla. Mix well.
Add in flour and mix together.
Fold in chocolate chips.
Once cookies are cooled, place a scoop of cookie dough on the middle of one cookie and spread it evenly.
Top the cookie dough with another cookie creating a “sandwich or whoopie pie”.