Preheat oven 350°F (175°C) with rack on lowest position in oven. Prepare pie crust. If using uncooked pie crust, prick the base a few times with a fork to prevent air bubbles before pouring in the filling.
When oven is hot, toast pecans on a baking sheet for 10 minutes. Roughly chop half of the toasted pecans. Reserve the other half for decorating the top of the pie.
Fill a small pot 1/3 of the way with water and bring to a boil over medium-high heat.
Reduce heat to low and place a heat-proof bowl on the pot. Fill the bowl with the chocolate and stir continuously until the chocolate is melted.
Add the butter to the chocolate, stirring well to combine until butter is melted. Mix in the cocoa powder until completely dissolved.
Take the bowl off the heat. Whisk in syrup, sugar, flour, bourbon, vanilla and salt.
Add the eggs and whisk well to combine. Fold in chopped pecans. Pour into prepared pie shell and top with remaining pecans.
Bake for 50-55 minutes or until centre is just set to the touch but still a little jiggly underneath. If your pie is browning too fast, loosely tent with foil.
The pie will be puffed up as soon as it comes out of the oven, but it will deflate and flatten as it cools. Let cool on a wire rack for about 4 hours and serve warm, or refrigerate to chill overnight.