-
Preheat oven to 350°F.
-
Whisk together flour, cocoa, baking soda, and salt in a bowl until combined.
-
Stir together milk and vanilla in a small bowl.
-
Beat together butter and brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 3 minutes
-
Add egg, beating until combined well.
-
Reduce speed to low and alternately mix in flour mixture and milk in batches, beginning and ending with flour, scraping down side of bowl occasionally, and mixing until smooth.
-
Spoon 2 tbsp. of batter about 2 inches apart onto 2 greased baking sheets.
-
Bake in upper and lower thirds of oven, switching position of sheets halfway through baking, until tops are puffed and cakes spring back when touched, 9 to 11 minutes.
-
Transfer with a metal spatula to a rack to cool completely.