Combine 4 ounces of butter, brown sugar and ½ cup evaporated milk in a large microwave-safe bowl. Cook on high for 1 minute. Remove from microwave and stir to combine. Return to microwave and cook on high for another 2-3 minutes or until mixture is no longer gritty (take a taste!).
Allow mixture to cool for 15 minutes, then transfer to the bowl of an electric mixer. Add 5 cups of powdered sugar and mix on low speed until incorporated. Increase speed to medium-high and mix for 1 more minute.
Add the 4 ounces of softened butter and vanilla. Mix on medium-high for 3-4 minutes or until light and very fluffy. Icing should be a nice, soft, spreadable consistency. If it's too thin, add a bit more powdered sugar, if too thick, add more evaporated milk - just a little at a time to thin. Frost cake as directed above.
Notes
This makes a large tall cake that will feed a crowd!