Pierce the sweet potato with a fork on all sides. Microwave it on high for about 4 minutes per side until it's very tender and the flesh is easily pierced with a fork. Let it stand for 5 minutes until cool enough to handle.
Meanwhile, in a medium microwave-safe bowl, add the butter, milk, maple syrup, kosher salt, and ground cinnamon. Microwave to heat through, 50-60 seconds. Don’t allow the mixture to boil.
On a cutting board, use your fingers to peel the cooked sweet potato. Roughly mash it with a fork right on the cutting board, then transfer the mash to the bowl containing the warm butter mixture.
Keep mixing and mashing with a fork (or a hand masher) until the mash is uniform and smooth, about 1 more minute. It won't be perfectly smooth unless you use a hand mixer or an immersion blender. But I like it a bit rustic. When done mixing, I like to use a rubber spatula to smooth it out.
Transfer the mash to a serving bowl. Serve immediately.
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Notes
If you can't find an extra-large sweet potato, you can use two smaller ones. Microwave the two of them together for about 3 minutes per side or until they can be easily pierced with a fork. They should be very tender.
To make a dairy-free version, replace the milk with well-stirred, full-fat canned coconut milk or plant-based heavy cream. Replace the butter with coconut oil (choose refined coconut oil if you don't want a slightly coconut-like flavor).
For a savory version, omit the maple syrup and cinnamon. Add ¼ cup of grated parmesan and ½ teaspoon of garlic powder. Both versions are excellent - it's up to you whether you prefer the sweet or the savory one.
I like a fairly rustic mash, so I use a fork to mash the sweet potato and a rubber spatula to smooth it out. For a smoother mash, you can use an electric hand mixer at the lowest speed.
You can keep the leftovers in an airtight container in the fridge for 3-4 days. Reheat them in the microwave, covered, at 50% power. You can also freeze this mash for up to three months. Thaw it in the fridge overnight. Once it's thawed, reheat it in the microwave.