Preheat oven to 375°F (190°C). Line baking sheets with parchment paper.

In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy (about 3-5 minutes with an electric mixer).

Beat in the eggs one at a time, then stir in the vanilla.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.

Stir in the chocolate chips and nuts (if using).

Drop by rounded tablespoons onto the prepared baking sheets, leaving about 2 inches between each cookie.

Bake for 10-12 minutes, or until the edges are set and the centers are still slightly soft. Let cool on baking sheets for a few minutes before transferring to a wire rack to cool completely.