To make the crust

1. In a bowl, whisk together the flour, sugar, salt and cinnamon. Add the cold butter, and rub it into the flour mixture until your have crumbs of varying size. You want to do this quickly so you don’t melt the butter with your fingers – you could also use a pastry cutter or potato masher.
2. Create a well in the mixture and pour in the buttermilk. Using a fork, bring the dough together, making sure all is combined. If needed, add a little extra buttermilk – you don’t want the mixture really wet, just moist enough to come together, it should still be shaggy.
3. Turn the dough out onto a lightly floured surface and knead until it comes together properly. Flatten into a disk, wrap in plastic and chill in the fridge for 1 hour.
4. You can use this chilling time to get started on the filling (below).
5. Line a 12×8″baking dish with parchment paper, with overhang at the sides
6. Remove the dough from the fridge and roll out on a lightly floured surface into a rough rectangle. You want it to be just slightly bigger than the baking dish.
7. Transfer it to the baking dish, pressing along the bottom. Trim the edges so it’s only lining the bottom – but don’t worry about leaving a little crust around the edges.
8. Cover with plastic wrap and return to the fridge for a minimum of 30 minutes.
9. Preheat the oven to 190C/375F

To make the filling

1. Peel the sweet potatoes and cut into small cubes. Put the potatoes into a large pot of water and bring to a boil. Simmer until potatoes are tender, about 20 minutes. Drain once cooked.
2. In the same large pot, add the potatoes, brown sugar, the spices, salt, butter and half of the evaporated milk. Cook on a low heat and use a potato masher to mash the potatoes as they cook and to combine the ingredients.
3. Simmer for 5 minutes. Use an immersion blender to make sure the mixture is as smooth as possible.
4. Once smooth, remove from heat and set aside in pot to cool slightly.
5. In a bowl, whisk together the remaining evaporated milk, caster sugar, eggs and vanilla. Whisk this mixture into the warm potato mixture until smooth and combined.
6. Pour the filling into the crust lined baking dish.
7. Bake in the oven for 10 minutes, during this 10 minutes make the streusel
8. Whisk the oats, flour, brown sugar, cinnamon and salt together in a bowl. Add the butter and rub into the flour mixture until you have large, coarse crumbs.
9. After the 10 minutes, remove the pie from the oven and reduce the temperature to 160C/325F
10. Sprinkle the streusel over the pie and return to the oven for 40 minutes, until the top is browned and the pie is set (you want it to have a little jiggle to it, but not loose).
11. Remove from oven and leave to cool on a wire rack.

Make the glaze

1. Place the butter in a medium saucepan and melt over a low heat.
2. Add the marshmallows and cook until melted, stirring constantly. Stir until smooth.
3. Remove from heat and whisk in the icing sugar and milk until combined and smooth
4. Add more sugar or milk to adjust to your desired consistency if necessary.
5. Leave to cool slightly and then either drizzle over the pie with a spoon, or place in piping bag and pipe in zig zags.
6. Remove the pie from the baking dish using the parchment paper overhang and cut into squares.