Preheat oven to 350°F (175°C). Line baking sheets with silicone liners or parchment paper.
Whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter and brown sugar until light and fluffy. Mix in the egg and vanilla.
Reduce mixer speed to low. Gradually add the flour mixture, mixing just until combined. Stir in the chocolate chips.
Drop the dough by 2-tablespoon portions onto the prepared pans, leaving about 2 inches between the cookies. Flatten the top of each cookie slightly.
Bake 12 to 14 minutes. The centers should still be slightly soft and not quite set. Be sure not to over-bake, or your cookies will be crispy and possibly a bit tough.
Cool the cookies on the pan for 5 minutes. Then transfer them to a wire rack to cool completely.
Notes
Store in an airtight container at room temperature up to a week.