In a bowl, mix the all-purpose flour and salt. Cut in the cold butter until the mixture resembles coarse crumbs. Add ice water gradually until a dough forms. Roll out the dough, fit it into a pie dish, and chill.

In a saucepan, combine dark brown sugar, cornstarch, and salt. Gradually whisk in milk and water. Cook over medium heat until thickened. Remove from heat and stir in egg yolks, followed by gelatin and butter until smooth.

Allow the filling to cool slightly before pouring into the crust. For the topping, beat the heavy cream with powdered sugar and vanilla extract until soft peaks form. Spread over the cooled filling and, if desired, sprinkle with toffee bits.