Cardamom Roasted Blackberries:
Preheat the oven to 450 degrees F.
Place the blackberries on a sheet pan and toss with sugar, cardamom, and vanilla bean seeds.
Roast in the oven for 20 minutes, tossing halfway through. Cool to room temperature before serving.
Crème Brûlée:
Adjust oven rack to lower-middle position and heat oven to 300 degrees F.
Combine 1 cup cream, sugar, and salt in a medium saucepan; scrape seeds from 1/2 vanilla bean into the pan, submerge pod in cream, and bring to a boil over medium heat, stirring occasionally.
Remove from heat and let steep for 15 minutes to infuse flavors.
Place a white kitchen towel in the bottom of a large baking dish and arrange four 4- to 5-ounce ramekins on the towel.
Bring a kettle of water to a boil.
After cream has steeped, stir in the remaining cream to cool down the mixture.
Whisk yolks in a large bowl until broken up and combined.
Slowly whisk about 1/2 cup cream mixture into yolks until loosened and combined; repeat with another 1/2 cup cream. Add the remaining cream and whisk until evenly colored and thoroughly combined.
Strain through a fine-mesh strainer into a 2-quart measuring cup; discard solids.
Pour or ladle mixture into ramekins, dividing it evenly among them.
Carefully place the baking dish with ramekins on the oven rack; pour boiling water into the dish, taking care not to splash water into ramekins, until water reaches two-thirds height of ramekins.
Bake until centers of custards are just barely set, about 30 to 35 minutes.
Transfer ramekins to a wire rack to cool. Cover tightly with plastic wrap and refrigerate until cold, at least 4 hours or up to 4 days.
Before serving, sprinkle each with about 1 teaspoon sugar; tilt and tap ramekin for even coverage. Caramelize the sugar with a torch and top with roasted blackberries.