Boil sweet potato whole in skin until fork tender, 20 to 30 minutes. Run cold water over sweet potato and remove skin. Mash in a large bowl. Pour in 60g melted butter and beat with an electric mixer until smooth. Blend in milk, cinnamon, vanilla, nutmeg, salt and egg. Pour into unbaked pastry case.
Combine golden syrup, brown sugar, 1 tablespoon melted butter and pecans; set aside.
Bake pie in the preheated oven for 20 minutes. Remove the pie from the oven and reduce the temperature to 180 C / Gas mark 4.
After 20 minutes, sprinkle pecan mixture over pie. Return pie to the oven and continue baking until set, about 25 minutes.