Using a small, sharp knife, split the vanilla bean in half lengthwise and scrape the seeds into a saucepan along with the heavy cream; add the scraped bean. Bring to a boil over medium heat, then immediately remove the pan from the heat and allow the mixture to cool for 10 minutes.
Combine the egg yolks and 1/2 cup sugar in a bowl, whisking until the mixture is very pale yellow. Whisking constantly, pour 1 ladleful of the warm heavy cream into the egg mixture to temper, then slowly pour the mixture back into the pan, whisking constantly.
Remove the vanilla bean and stir in the mascarpone. Chill the mixture, covered, for 1 hour or up to overnight.
Preheat the oven to 325 degrees F. Divide the batter into four 5-inch or six 4-inch ramekins. Place the ramekins in a larger roasting pan and fill the pan with hot water, coming halfway up the sides of the ramekins. Bake until set like gelatin and a tester inserted in the centers comes out clean, 30 to 45 minutes.
Remove the ramekins from the water bath and let cool at room temperature, 30 minutes. Transfer to the fridge until completely chilled, about 1 hour.
Sprinkle the remaining 2 to 3 tablespoons sugar on top of the creme brulees (you want a nice, even layer).
Choose a metal spoon that you don't love so much and place it over a high flame on the stove. Let the bowl of the spoon get hot enough that it looks red.
Remove the spoon from the stove (the handle shouldn't be hot because flatware doesn't conduct heat) and burn the top of the creme brulee with it. Reheat the spoon for each creme brulee, cleaning the spoon after each use.