Roast at 400F for ten minutes then flip the berries and roast for an additional 10 minutes. Let cool for at least ten minutes before attempting to puree them.
When cooled add them to a food processor until a smooth puree is formed. Now you could use this as it is now or you can reduce it to concentrate the flavor.
Add half of the dry mixture, mix to combine, then add half of the buttermilk and repeat the process one more time. Make sure to not over mix the cupcake batter