Preheat the oven to 400 degrees F.
Cut the potatoes in half length-wise on a cutting board with a large sharp knife . Take out a sheet pan and place a piece of parchment paper on it, covering the surface. Place the halved potatoes on the sheet pan, flesh side up.
Measure out the oil with a measuring spoon and pour a small amount onto each potato half, until the measuring spoon is empty. Using your hands, coat each potato half with the oil, both skin side and flesh side.
Place the potato halves flesh side down on the sheet pan and cook in the oven for 45 minutes, until a fork or knife inserted into each potato goes in without resistance.
While potatoes are cooking, take out a 9x13 (or similar size) dish and give it a thin coat of butter. Set aside.
Next make the topping. Combine the ingredients for the topping into a large bowl, measuring the right amount with measuring cups and spoons, and mix with a wooden spoon until well combined. Set aside.
Once potatoes are done cooking, take them out of the oven and let sit for a couple minutes to cool. As they cool, start preparing the rest of the potato mixture. Turn the oven temperature down to 375 degrees F.
In the bowl of a stand mixer (or in a large bowl if using a hand mixer), measure out and combine the remaining ingredients from the sweet potato mixture list with the whisk attachment (or a hand mixer).
Once the potatoes have cooled enough to touch, peel the skins off using your hands. They should peel easily if they are cooked all the way through.
Place the peeled potato pieces in the bowl of the stand mixer and whisk everything together until there are no lumps of potato left.
Scoop the sweet potato mixture into the 9x13 dish with a rubber spatula and spread into an even layer.
Spoon the topping onto the sweet potato mixture with a wooden spoon and spread that into an even layer as well. Place the baking dish into the over and cook for 35 minutes. The dish is done when the topping is golden brown on top.