Alcohol evaporation of beer in a cake
Alcohol never all evaporates unless you evaporate all the water as well. The actual amount per slice is minimal anyway - equivalent to about a shot of beer in a portion, in the recipe I use. If even that's too much for you, start with de-alcoholised beer. You may have trouble finding alcohol-free stout. 

From the USDA Table of Nutrient Retention Factors Release 6 quoted in merl's answer to a related question, baking reduces the alcohol to 25% in an hour and 10% in 2 hours. Quite a lot of water will evaporate as well during that time.