Put your cast iron pan (use a 9-10 inch pan, at least--for best "fluff") onto the top rack of the preheated oven. We want the pan to be hot when we add the batter.
While the berries are roasting and the pan is warming in the preheated oven, crack the eggs and pour them into your blender container. Add the milk, 1 tbsp. of butter, flour, salt, sugar, cardamom and rosewater (if using). Blend until smooth. The batter will be pretty thin--similar to a crepe batter.
Let the batter rest for 5 minutes (use this time to check on your berries--give them a good stir) and then remove your warmed cast-iron pan from the oven. Place it on the stovetop over high heat. Tip in 2 tbsp. of butter.
Once the butter melts and begins to bubble (but not brown!), pour the blended batter into the hot pan. After you've poured it all in, quickly move the pan onto the top rack of the oven. Make sure you use oven mitts--don't burn yourself on the pan handle.
Set the timer for 12 minutes and leave it alone. After 12 minutes, check it. When it's done, then center will be puffed--it might look crazy puffy, don't worry, that's normal--and the edges will be golden brown. If it's only lightly browned, let it cook a few more minutes. Your Dutch baby could take 12-20 minutes, depending on what size pan you used.
When the Dutch baby is done, carefully remove the pan from oven. Also remove the pan with the roasted blackberries. Using a spatula, transfer the cooked Dutch baby onto a baking sheet or a large plate--being careful not to burn yourself.
Stir the blackberries, mashing lightly with the back of a spatula to create a jam-like consistency. Dust the pancake with some powdered sugar and then top with the cardamom roasted blackberries.
Cut the pancake into 8 slices (like a pizza) and enjoy!