Preheat to 300°F; whisk flour, cocoa, baking powder, soda, and salt.
Microwave 6 oz chocolate with oil and butter at 50% power, stirring until smooth; whisk in milk.
Beat sugar and eggs until pale, 4 min; whisk in chocolate; fold in dry mix and reserved chocolate.
Scoop 20 heaping tablespoons onto sheets; bake 16 min, rotating halfway.
Cool on sheets 30 min; transfer and serve.