Adding it up front may give you more caramelized flavors versus adding it towards the end. Probably not a lot. DME is messy stuff; the steam coming off the pot is more than enough to make the whole bag a sticky, chunky mess of doughballs that you have to smash against the side of the pot to mix in. It may help you to mix the DME with cold water until it's vaguely pourable and then add this to the boiling pot. You picked the perfect style of beer to start with; stouts are very forgiving.
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