In a small saucepan, whisk together the coconut milk (both cans), sugar, lime zest, agar powder and salt. Place the saucepan over medium heat, and continue whisking until the mixture is completely smooth. Bring the coconut milk to a simmer, stirring occasionally with a rubber spatula. Continue simmering until the milk thickens slightly and lightly coats the spatula (it will still look thin, but it should leave a line when you swipe the spatula with your finger—check out the video above for reference), about 4-5 minutes longer.
Pour the liquid into a large measuring cup or a bowl with a spout (this will make it easier to pour it into the ramekins). Divide the panna cotta mixture between the prepared ramekins. Chill, uncovered, until set, about 1 hour. Do Ahead: The panna cotta can be made up to 2 days in advance. Once the panna cotta has set, cover the ramekins with plastic wrap.
To serve, run a sharp knife around the edge of a ramekin; invert the ramekin onto a plate; and shake firmly up and down until the panna cotta slides out (you can also serve the panna cotta in the ramekins with the fruit spooned on top). Spoon the roasted berries and their juice over the panna cotta, and serve!