Fill a large pan with boiling water from the kettle and set over a medium-high heat. Carefully drop in the whole, unpeeled sweet potatoes and cook for 30 minutes until very tender when pierced with the tip of a sharp knife. Drain well and leave to cool for 10 mins, then peel.

Heat the oven to 180C/160C fan/gas 4. Tip the peeled sweet potatoes into a large bowl, add the butter and mash with a potato masher until completely smooth. Tip in the brown sugar, milk, eggs, spices and vanilla and whisk until everything is combined and smooth.

Lightly butter a 20 x 30cm baking dish, then scrape in the sweet potato mixture. To make the topping, rub the butter into the flour, then stir in the brown sugar, pecans and a good pinch of salt. Sprinkle the topping evenly over the sweet potato filling, then bake for 45-50 mins until golden on top.