1. Preheat and prep. Preheat the oven to 350° F. Coat your baking pan with cooking spray or butter it well. You can also line the bottom of the pan with a piece of parchment or wax paper. Especially if you plan to flip it out and cut individual servings, the parchment makes the process much easier. 
2. Peel and slice the potatoes. Peel the sweet potatoes well with a vegetable or y-shaped peeler, and cut off any woody or pointed ends. Aim for slices ⅛"-thick. A mandolin is a helpful tool (but be careful!). A sharp chef's knife gets it done.
3. Season the heavy cream. Whisk together the heavy cream and maple syrup in a small bowl or measuring glass. Measure the salt out in a small container. 
4. Layer the potatoes in the pan. Shingle the sweet potato slices across the bottom of your baking pan. Then drizzle a few tablespoons of the cream mixture and sprinkle a couple of pinches of the salt evenly over the layer. Repeat, making the layers as flat as possible and drizzling each with a little of the cream mixture and a sprinkling of salt. By the time you place the last layer, you should've used all the cream mixture and the entire two teaspoons of salt. 
5. Bake. Bake the gratin until a knife easily pierces all the way to the bottom, between 55 and 65 minutes. If the top layer begins to brown too much before the potatoes are tender, cover it with foil. If you want the top more browned and crispy, you can place the baked gratin under the broiler for a few minutes. Serve the gratin garnished with plain yogurt, crème fraîche, or a dusting of cinnamon or warm spices.
6. Storage. Store any leftovers in the refrigerator for up to a week. Reheat the gratin in a 350° F oven until hot in the center. 

For Individual Portions

1. Weight and cool. Press a piece of greased foil or parchment paper on top of the still-hot gratin. Then set a plate, slightly smaller pan, or removable tart/cake pan bottom on top that fits just inside the edges of the pan. Set a few canned goods or a similarly heavy item on top. Cool the gratin completely in the refrigerator, overnight if you like.
2. Loosen and flip. Run a butter knife around the outside of the gratin to loosen it from the edges of the pan. Place a plate, sheet pan, or cutting board on top of the pan. Then holding both tightly flip them over quickly to pop the gratin out. If the gratin doesn't immediately fall out of the baking dish, tap the bottom of the pan (or just give it a few moments). Place a cutting board upside-down on the freed gratin, and flip again to get it right-side up. 
3. Cut, warm, and serve. With a cookie cutter or a knife, cut the gratin into your preferred shape. I like to use a pastry mold or cookie cutter, but a knife works just as well for creating squares or triangles. Place the individual portions on a greased or parchment-lined sheet pan and warm them in a 350° F oven until hot, about 10 to 15 minutes.