What ingredients in chocolate make it melt faster than others?

It pretty much depends on how much cocoa butter is in the chocolate. The more cocoa butter the longer it takes to melt the solid into a liquid form. Dark or bakers chocolate has less cocoa butter than milk chocolate.

Here is a breakdown of types of chocolate and their makeups

Depending on what is added to (or removed from) the chocolate liquor, different flavors and varieties of chocolate are produced. Each has a different chemical make-up, the differences are not solely in the taste. Be sure, therefore, to use the kind the recipe calls for, as different varieties will react differently to heat and moisture.

* Unsweetened or Baking chocolate is simply cooled, hardened chocolate liquor. It is used primarily as an ingredient in recipes, or as a garnish.

*Semi-sweet chocolate is also used primarily in recipes. It has extra cocoa butter and sugar added. Sweet cooking chocolate is basically the same, with more sugar for taste.

* Milk chocolate is chocolate liquor with extra cocoa butter,
sugar, milk and vanilla added. This is the most popular form for chocolate. It is primarily an eating chocolate.

* Cocoa is chocolate liquor with much of the cocoa butter removed, creating a fine powder. It can pick up moisture and odors from other products, so you should keep cocoa in a cool, dry place, tightly covered. There are several kinds of cocoa:
~ Low-fat cocoa has the most fat removed. It typically has
less than ten percent cocoa butter remaining.
~ Medium-fat cocoa has anywhere from ten to twenty-two percent cocoa butter in it.
~ Drinking or Breakfast cocoa has over twenty-two percent
left in it. This is the cocoa used in chocolate milk powders like Nestle’s Quik.
~ Dutch process cocoa is cocoa which has been specially
processed to neutralize the natural acids in the chocolate. It is slightly darker and has a much different taste than regular cocoa.

* White chocolate is somewhat of a misnomer. In the United States, in order to be legally called ‘chocolate’ a product must contain cocoa solids. White chocolate does not contain these solids, which leaves it a smooth ivory or beige color. Real white chocolate is primarily cocoa butter, sugar, milk and vanilla. There are some products on the market that call
themselves white chocolate, but are made with vegetable oils instead of cocoa butter. Check the label to avoid these cheap imitations. White chocolate is the most fragile form of chocolate; pay close attention to it while heating or melting it.

* Decorator’s chocolate or confectioner’s chocolate isn’t really
chocolate at all, but a sort of chocolate flavored candy used for things such as covering strawberries. It was created to melt easily and harden quickly, but it isn’t chocolate. If you want quick and easy, use decorator’s chocolate. If you want the real thing, use real chocolate and patience.