When we talked about mixing ingredients to make various types of chocolate earlier, this is how it works:
Dark chocolate is the blending of ingredients - sugar, cocoa butter, cocoa liquor, and (sometimes) vanilla.
Milk chocolate: sugar, cocoa butter, cocoa liquor, milk or milk powder, and vanilla
White chocolate: sugar, cocoa butter, milk or milk powder, and vanilla
Any guesses as to which chocolate would melt the fastest based on this information? You guessed correctly: dark chocolate!
Dark chocolate melts the fastest... It's because dark chocolate is in its purest form. Since different ingredients have different melting points, adding additional ingredients like milk or milk products, sugar, fats, and other things reduce melting.