1. To turn from a solid into a particular kind of liquid or liquid-like state, as by a result of excess heat. My chocolate bar melted into goop after I left it in the car all day. OK, everyone grab a slice of the ice cream cake before it melts into mush!
2. To cause something to turn from a solid into a particular kind of liquid or liquid-like state. In this usage, a noun or pronoun is used between "melt" and "into." We need to melt this ore into a molten state so that it can be molded into the desired shape. We used salt to melt the ice on the roads into water.
3. To cause something to melt and combine with a mixture or other liquid. In this usage, a noun or pronoun is used between "melt" and "into." After you've melted the butter into the batter, you should notice it beginning to fluff up. The chemical reaction causes the salt to melt into the compound.
melt something into something
to cause something to change its state when melting. The ice melted into a cold liquid that we could drink. We melted the fat into a liquid that we could deep-fry in.
melt into something
to melt and change into a different state. All the ice cream melted into a sticky soup. The candles melted into a pool of colored wax in all the heat we had last summer.