Prepare the croissants by using a serrated knife to cut a small pocket into the side of each croissant, being careful not to cut all the way through. This creates a space for the custard filling while maintaining the integrity of the pastry.
Make the custard filling by combining heavy cream and vanilla bean (both seeds and pod) in a saucepan. Heat over medium-low until steaming but not boiling, then remove from heat and allow the mixture to steep for 15 minutes.
Whisk the egg yolks with sugar and salt in a medium bowl until the mixture becomes pale yellow and slightly thickened. Gradually stir in the cornstarch until completely incorporated.
Temper the egg mixture by slowly pouring about 1/2 cup of the warm cream into the egg mixture while whisking constantly. This prevents the eggs from curdling when added to the hot cream.
Return the entire mixture to the saucepan and cook over medium-low heat, stirring constantly with a wooden spoon or heat-resistant spatula, until the custard thickens enough to coat the back of the spoon (about 5-7 minutes).
Strain the custard through a fine-mesh sieve into a clean bowl to remove the vanilla pod and any lumps. Cover with plastic wrap, pressing it directly onto the surface of the custard to prevent a skin from forming, and refrigerate until completely chilled (at least 2 hours).
Fill the croissants by transferring the chilled custard to a piping bag (or use a small spoon) and carefully pipe the custard into the pocket of each croissant. Be generous but careful not to overfill.
Prepare for brûléeing by sprinkling a thin, even layer of turbinado or demerara sugar over the top of each filled croissant. The coarser sugar creates a better caramelization than regular granulated sugar.
Caramelize the sugar using a kitchen torch, moving it continuously across the surface until the sugar melts and turns a deep amber color. If you don’t have a torch, place the croissants under a broiler set to high for 1-2 minutes, watching carefully to prevent burning.
Serve immediately while the caramelized sugar is still crisp and the contrast between the warm, crackling top and cool, creamy filling is at its most magical. These are best enjoyed fresh!