Begin by making the chocolate pastry case for the derby pie. In a food processor, mix together 1 cup plain flour, cocoa powder, and powdered sugar until they are well combined. Then, add ½ cup cold, cubed butter and allow the food processor to run until the mixture looks like fine sand. Finally, add 1 large egg to the processor, and mix until a soft dough ball has formed. Remove and cover the dough and place it in the fridge for at least 2 hours.
Preheat the oven to 350 F. Lightly flour a clean surface and roll out the chocolate dough until it is roughly ¼-inch thick. Carefully transfer the rolled dough to a 9-inch pie or tart tin. Press the pastry into the tin, trim the edges, and prick the bottom of the pastry with a fork.
Cover the pastry with a layer of baking paper and fill it with baking beans. Blind bake the pastry case for 25 minutes until it is just cooked through. Remove it from the oven and set it aside to cool.
To prepare the filling, melt 4 tablespoons butter with the chopped dark chocolate, either in a microwave or over a Bain Marie.
In a separate bowl, whisk together the maple syrup, brown sugar, vanilla extract, 2 eggs, and salt. Then, mix in the bourbon and 1 tablespoon plain flour before pouring in the melted chocolate mixture. Whisk it all quickly to create a smooth batter.
Fold the chocolate chips into the batter and pour the batter into the prepared pastry shell. Add the pecan halves on top. Bake the pie in the oven for 20 minutes, until the edges are set and the middle is still a little wobbly. Allow the pie to cool for a few hours to fully set before slicing and serving.