How should I melt a milk chocolate bar to spread on cake

Why don't you put the bar (in a few pieces) on top of the hot cake when it comes put of the oven and spread?

I might whip in some butter with an electric mixer in addition to the heavy cream if you want something more frosting-like.

also, since it looked like you were asking about the symphony chocolate bar (a few minutes before this one), make sure it doesn't get too runny, or the toffee/almond pieces that make a symphony bar so good won't stay put.

my advice would be just to break it up so it melts faster and then use a double boiler (or bowl over saucepan of simmering water) to slowly melt it. it works for chocolate chips, and I'm sure it would melt your chocolate bar. however, i don't know how well a pure chocolate bar would spread on a cake...so you might need to add a little heavy cream to thin it out a little so you don't tear your cake as you try to spread it. I would add it by tablespoon until it reaches the consistency you need it to be. also, this recipe link as a melted chocolate frosting/icing...and so this might help you with proportions of chocolate to cream.

http://www.foodnetwork.com/recipes/ina-garten/chocolate-ganache-cupcakes-recipe/index.html

hope your cake turns out as a success!