I am using the Planet Cake chocolate mud cake recipe in an 8 inch square pan as recommended. Cooking time is 1 hour 40 mins in a 160 degree oven. I put the cake on the middle shelf and don't open the door until the time is up. The cake is cooked all the way through although probably a little more moist just under those bubbles.
I noticed when I made the cake in a round tin slightly smaller than the recommended 9 inches for the recipe the cake rose beautifully and had no bubbles.
When I mix the chocolate sauce and egg/oil/buttermilk mixture into the dry ingredients bubbles form. I mix slowly with a large spoon as the recipe suggests so I can't be over mixing. I have tapped the tin on the counter so air bubbles can rise up and out but many remain on the surface. Should this happen? I have attached a photo.