I have really dark hair, so this is my go-to recipe. You can add more or less cocoa powder as needed - I use two tablespoons every time:
1/4 cup cornstarch
1 tablespoon baking soda
2 tablespoons cocoa powder
Mix everything together in a small jar and you're ready to go. Special bonus with this one: smelling like chocolate. :D
Just be careful that you you don't add so much cocoa powder that it gets really dark - you need to keep the amount of cocoa powder less than the amount of cornstarch and baking soda, otherwise the heat from your hands and head can warm the cocoa powder and create a melty mess.