There are several ways to flavor this dark chocolate and several ingredients to avoid. Avoid ingredients with any water-content including water-based flavorings. Alcohol-based extracts are also a risk - although I admit I've used vanilla extract with this homemade dark chocolate before with no issues - but using vanilla powder is less risky.  It's best to use oil-based flavorings, often called 'candy/chocolate flavoring'. You can also use powdered ingredients such as spices: cinnamon, pumpkin spice, ginger, etc. You can also mix in other dry ingredients such as dehydrated fruit pieces, nuts, and crushed snacks (biscuits like gingerbread, pretzels, etc.).