For crust, combine flour, ground pecans and salt. Cut in butter until an even, crumbly texture. In a separate bowl, whisk brown sugar with egg yolk and 4 Tbsp water and add to flour. Mix just until dough comes together, adding a little more water, if needed. Wrap and chill for 30 minutes.
On a lightly floured surface, roll out dough to 1/8-inch thick. Line a 10-inch pie shell with dough and trim. Chill for 20 minutes.
Preheat oven to 375° F. Line pie shell with aluminium foil and weight with pie weights, rice or dried beans. Bake for 18 to 20 minutes, then remove foil and weights and bake for 5 to 7 minutes more, until centre of pie shell is dry and edges are lightly browned. Allow to cool.
For filling, line bottom of baked pie shell with toasted pecan halves. In a saucepot, mix brown sugar and flour together, crushing any visibly large lumps of brown sugar. Whisk in milk until smooth. Stir filling over medium low heat to start dissolving sugar, then stir in butter. Cook filling for about 8 minutes, stirring constantly, until it thickens. Remove from heat and scrape into a bowl. Stir in vanilla and egg yolk. Scrape filling into pie shell and spread evenly. Chill for at least 2 hours before topping.
For topping, whip cream to soft peaks and fold in sugar and vanilla. Spread evenly over butterscotch filling and garnish with chopped pecans. Chill until ready to serve.
Pie with filling can be made up to 2 days in advance, and topped up to 6 hours in advance.
Yield: 1 10-inch pie. Makes 8 to 12 servings