Prick 1 large sweet potato and roast at 400 degrees F for 50 to 60 minutes, until fully collapsed and a knife meets no resistance. Do not boil it.

Scoop out the flesh, mash it completely smooth, measure 1 cup and cool to room temperature.

Whisk the flour, brown sugar, cinnamon and salt in a small bowl. Add the cold cubed butter and rub it in with your fingertips until the mixture holds in clumps from crumb size up to pea size.

Stir in the chopped pecans and refrigerate the bowl while you mix the batter.

Heat the oven to 350 degrees F. Grease a 9x5 inch loaf pan and line it with a parchment sling.

Whisk the flour, baking powder, baking soda, salt, cinnamon, ginger and nutmeg in a medium bowl.

In a large bowl, whisk the cooled sweet potato mash with both sugars, then whisk in the oil until glossy. Add the eggs one at a time, then the milk and vanilla.

Fold the dry ingredients into the wet with a spatula, about 15 strokes, stopping the moment the flour disappears. The batter should be thick enough to mound.

Scrape the batter into the pan and level it. Spoon the chilled streusel evenly over the top, all the way to the edges, without pressing it down.

Bake 60 to 70 minutes, tenting loosely with foil at 40 minutes once the pecans have colored. Test with a skewer angled into the crumb from the side rather than through the topping.

Cool in the pan 20 minutes, then lift out by the parchment and cool at least 1 hour on a rack before slicing.