I'm looking for a Not too sweet chocolate frosting/buttercream that will go with yellow cake! I tried a few recipes and one tasted like powder sugar the other I used milk chocolate and it tasted weird.

Try using melted unsweetened chocolate.

What about whipping cold dark chocolate ganache? Its still just chocolate and cream.

I just use a basic buttercream recipe. but I read here on CC to add in a store bought can of frosting, I'll use milk chocolate. Makes it a nice dark color to.

I like that idea cookiemama2. I had never seen that idea before.

Here is my go-to chocolate buttercream:

1 stick butter, room temp.
2/3 cup cocoa powder
3 cups PS
1/3 cup whole milk
2 tsp. pure vanilla
1/4 tsp. salt ( I use white popcorn salt)

Beat butter and cocoa powder until well combined. Add the rest of the ingredients and mix until the icing lightens in color a bit and is fluffy. Add more or less milk as needed. Everyone raves over this buttercream and I double and triple the recipe when I do bigger cakes.

This one's really rich
http://allrecipes.com//Recipe/chocolate-fudge-buttercream-frosting/Detail.aspx