Line baking sheets with parchment paper or lightly grease them.
In a medium bowl, whisk together 2 ½ cups all-purpose flour, ¾ cup unsweetened cocoa powder, 1 teaspoon baking soda, ½ teaspoon baking powder and ½ teaspoon salt. Set aside.
In a large bowl, use an electric mixer to cream 1 cup butter, softened, and gradually add 1 ½ cups sugar, mixing until light and fluffy, about 3-4 minutes.
Beat in the wet ingredients: 1 egg, 1 tablespoon milk, and 1 teaspoon vanilla extract until well combined.
Combine the wet and dry ingredients. Gradually add the flour mixture to the butter mixture, mixing on low speed until just combined. Do it in 2 or 3 parts, but don't wait until it's fully incorporated to add the next part, so you don't overmix the dough.
Roll about 1-inch dough balls and roll in sugar (optional). Place them 2 inches apart on the prepared baking sheets. You can also drop spoonfuls of dough if preferred and omit the rolling.
Bake for 8-10 minutes, or until the edges are set. The centers may look slightly underdone; this is normal for fudgy cookies. Smack the pan against the counter if you want chewier cookies, just after you take them out of the oven. They will deflate.
Let cookies cool on the baking sheet for 5 minutes before removing carefully with a spatula and transferring them to the wire rack to cool completely.
Notes
You can bake the cookies right after mixing the dough or chill it first, for several hours or a day or two. The difference is in consistency and sometimes flavor. The chilled dough expands less and can have a more rounded flavor as the ingredients have time to meld together. In my experience, the difference is not that big, and I usually bake half and then chill the rest.To roll or not to roll in sugar before baking? It's your choice. Rolling the cookie dough balls in granulated sugar creates a crunchy top and adds sweetness. If you don't, you'll still get slightly crackly cookies with a slightly more intense chocolate flavor. If in doubt, test both ways with a few cookies and decide before you bake the whole batch. Double dose of chocolate: add about ½ cup of semisweet chocolate chips together with the last part of the flour.