I did this recipe before.
5.00 lb Light Dry Extract (17.5 SRM)
1.00 lb Flaked Oat
0.50 lb Roasted Barley (300.0 SRM)
0.75 lb Chocolate Malt (350.0 SRM)
1.5 oz Goldings, East Kent [5.00 %] (60 min)
It has a little more roasted barley flavor than guinness so I might drop the Roasted Barley down to a .25 lb to see if it matched up to Guinness, otherwise it was a winner for me.