For the pastry, lightly grease a 28cm/11in fluted loose-bottomed flan tin. Combine the flour, butter and icing sugar in a food processor and pulse until the mixture resembles fine breadcrumbs. Add the egg yolks and a splash of milk and pulse until the pastry just comes together. Bring the dough into a ball and wrap in cling film. Freeze for 10 minutes, then place in the fridge for a further 20 minutes.
Bring a pan of salted water to the boil and add the sweet potatoes. Cook for ten minutes or until very tender. Drain and allow to cool slightly.
When the pastry has chilled, roll it out to a thickness of ½cm/¼in, then transfer it to the greased tin, leaving a little pastry dangling over the edge. Use any leftover pastry to patch-up any holes. Press the chopped pecans into the pastry. Prick the base with a fork and chill in the fridge for 20 minutes.
Preheat oven to 170C/375F/Gas 5.
Cover the pastry case with baking parchment and fill with baking beans. Bake blind for 10 minutes, then remove the paper and beans and bake for a further 15 minutes. Cool the pastry case in the tin on a wire rack.
Mash the cooked sweet potato very roughly. Stir in the sugar, eggs and spices. Then add the condensed milk, vanilla and butter and mix thoroughly.
Pour the filling into the cooled case. Arrange the pecan halves on the top of the filling and sprinkle with ground cinnamon.
Bake for 25 minutes, or until the filling is set. Leave to cool for 10 minutes, before removing the pie from the tin. Aloow the pie to reach room temperature before transferring to the fridge to chill.