1In a medium saucepan set over medium-high heat, combine maple syrup, brown sugar, corn syrup, and salt and bring to a boil, stirring until sugar dissolves. Continue boiling for a minute, reducing heat if it looks like it might boil over. Remove from the heat and stir in the bourbon, then the butter; whisk until butter melts. Let cool to lukewarm, whisking occasionally, about 20 minutes.
2Preheat oven to 350°F. Brush crust with the beaten egg white, and scatter with hazelnuts. In a medium bowl, whisk together the eggs and vanilla; whisk in the cooled maple-sugar mixture. Pour into the crust, over the hazelnuts. Bake for 45-50 minutes, until filling is set and slightly puffed.