Preheat oven to 350 degrees.
Line 2 sheet pans with parchment paper. Set aside.
In a large bowl, sift together flour, cocoa powder, baking soda, and salt. Set aside.
In the bowl of an electric mixer, beat butter and sugars until light and fluffy, about 3 minutes.
Beat in egg.
Stir in half of flour mixture.
Mix in milk.
Add remaining flour mixture.
Stir in vanilla, peppermint oil, and chocolate chunks.
Drop by scoopfuls onto prepared baking sheets.
Bake for 12 minutes.