1/2 cup (1 stick) unsalted butter, room temperature
1 cup packed dark brown sugar
1 large egg
1 teaspoon vanilla extract
1/2 cup all-purpose flour
1/3 cup cocoa powder
1/2 teaspoon salt
1/4 teaspoon baking soda
1 1/2 cups rolled oats
1 cup dark chocolate chips (semi-sweet are also good)
Preheat the oven to 350 degrees Fahrenheit. Line a baking sheet with a silicone mat (a non-stick sheet will also work just fine, I just like the ease of the silpats).
In the bowl of an electric mixer fitted with a paddle attachment, or in a large bowl using a hand mixer, beat together the butter and the brown sugar until pale yellow and fluffy, about three minutes.
Add egg and vanilla, and beat until blended.
In a medium bowl, whisk together the flour, cocoa powder, salt, and baking soda.
Add to the wet ingredients in the mixer bowl and mix just until combined. Add rolled oats and chocolate chips and mix just until combined, scraping down the sides as needed.
Scoop by tablespoonful onto your cookie sheet, spaced about two inches apart.
Bake for about 10 minutes, until cookie appears set and no longer wet. They might not look completely done, but they'll set up after they cool! If you like a crisper, less chewy cookie, let them cook for 2 - 3 minutes longer.
Remove from oven and let sit for about 5 minutes, then move to a wire rack to cool completely.
They keep nicely for 4 - 5 days in an airtight container at room temperature, or you can freeze them in a zipper bag for several months.