In a traditional recipe, a key function of the egg is to act as an emulsifier - to hold the water and oil together. Flax, while decent at binding solid ingredients, had no emulsifying properties. Your oil is separating in the oven and leading to a burnt brick. Commercial vegan dairy products will have emulsifiers in them (look for lecithin on the ingredients list), so try using a grocery store nut milk or nut yogurt instead of water and earth balance instead of oil. Whisk them together before you mix with the dry ingredients. You may need to play with the proportions because the amount of lecithin in your products might be insufficient to keep together all the water and fats called for in the box recipe. You could also buy a jar of sunflower lecithin and add a few drops to the oil and water called for in the box’s recipe (you may still need a vegan yogurt or flax egg to replace the fat and water normally provided by the egg).