How would you make this cut?

I'm looking to build shelves beside my fireplace but need to trim my mantel width down to accomodate them.

I have access to a circular saw, a reciprocating saw and a jig saw. I could also get a router if needed.

This is from a diy website but they had this part done by a professional. I'm just curious how you would go about cutting this. What tools should I use?