Unsweetened cocoa is rich, has a stronger bitter taste, and is also acidic, whereas unsweetened Dutch process cocoa powder (also known as European cocoa powder) takes the acid out of the cocoa which reduces the bitter aftertaste. If you only have regular unsweetened cocoa powder at home, I personally think it’s okay to still use that and just increase the sweeter a little bit. The vanilla extract also helps to add a punch of flavor to balance out the chocolate. I've used both types of cocoa powder and both work great!