Preheat oven to 400°F and lightly butter an 11 inch oval baking dish (see notes for alternatives). You want something that will fit the 8 rolls comfortably with a little room between them to expand.
To make the biscuit dough, place the flour, brown sugar, baking powder, baking soda, salt, nutmeg, and cinnamon in the bowl of a food processor. Pulse a few times to mix the ingredients together.
Add the cold butter to the food processor, and pulse until the mixture is coarse and crumbly. A few pea-size pieces of butter remaining are fine.
Add the vanilla extract to the milk, and drizzle it through the top of the food processor while pulsing until the dough comes together into a single mass.
Remove the dough to a lightly floured surface and knead it just about 10-12 times until it's smooth and not so sticky.
Roll the dough out into a single rectangle about 18 inches long and 12 inches wide.
While your berries are still frozen, quickly toss them with the sugar and the cornstarch to coat evenly. Sprinkle this mixture over the rolled out biscuit dough.
Gently roll the dough from the long edge into a long tube.
Using a clean piece of dental floss (this works best), or a sharp knife, cut the log into 8 equal size rolls.
Carefully move the rolls to your prepared baking dish, spreading them out evenly. They will likely be delicate, so move them gently. If any berries fall out, just tuck them back into the rolls.
Turn the oven temperature down to 375F, and bake for about 35-40 minutes until puffed, golden, and the juices of the blackberries are starting to bubble.
Meanwhile, whisk together the glaze ingredients until smooth.
When the rolls come out of the oven, allow them to cool briefly, and then spread the glaze over them while they are still warm.
Notes
You can also make these rolls in a 9x13 baking dish, or any baking dish you have that fits the 8 rolls comfortably -- you want about 1/2 inch between the rolls to leave room for them to expand as they bake.