For the sweet pastry, grease a 28 cm removable base tart tin.
Place butter into a mixer fitted with a paddle attachment. Add icing sugar and mix until creamy then add the icing, flour and salt. Mix for 30 seconds and add the egg, continuing to mix for 2 minutes until it comes together into a ball. Tear off 2 sheets of baking paper 30cm x 30cm. remove mixture from the bowl and roll out between baking paper sheets to 3 mm thick and then line the tin. Do this by peeling off one sheet of the paper, turn upside down and lay gently into the tart tin. Remove paper and mould the pastry up the sides of the tin with your finger tips. Remove any excess pastry with your thumb by pressing down along the edge of tin. Allow to rest for 30 minutes.
Take one of the sheets of paper, press into the tart tin on top of the pastry, fill with rice or baking beans and blind bake in the oven for 15 minutes until golden. Remove from the oven, take out baking beans and cool. Turn oven temperature down to 130°c.
Combine the mashed sweet potatoes, both sugars, butter, vanilla, egg, cinnamon, nutmeg, allspice and cream in a large mixing bowl.
Beat with an electric mixer until well blended and smooth.
Spread the mixture into the pie shell and sprinkle the pecans evenly over the top.
In another large mixing bowl, prepare the topping. Combine the granulated sugar, eggs, corn syrup, melted butter, salt, cinnamon and vanilla. Stir until well blended and smooth.
Spread the mixture over the pecans. Bake until the pie is nicely browned, about 1 ½ hours.
Let cool completely on a wire rack and serve topped with whipped cream.
