With the rack in the middle position, preheat the oven to 350°F (180°C). Line the bottom of an 8-inch (20 cm) springform pan with parchment paper.
In a food processor, chop the pecans with the brown sugar. Add the butter and pulse until the mixture comes together. Press the mixture over the bottom and two-thirds up the sides of the springform pan.
Bake for 18 minutes or until the crust begins to brown. Let cool completely on a wire rack.
In a pot off the heat, combine the brown sugar and cornstarch. Whisk in the egg yolks and half of the milk (1/2 cup/125 ml) until smooth. Whisk in the remaining milk and the cream. Bring to a boil over medium heat, whisking constantly and scraping the sides and bottom of the pan until the mixture has thickened. Simmer for 1 minute, whisking constantly. Remove from the heat. Whisk in the bourbon and vanilla. Strain through a sieve. Stir in the butter.
Spoon the butterscotch custard into the cooled pecan crust. Cover with plastic wrap. Let cool, then refrigerate for 6 hours or until completely chilled.
When ready to serve, unmould and sprinkle lightly with fleur de sel.