Preheat oven to 375 degrees.
Generously grease a standard sized muffin tin.
In a small microwave-proof bowl, combine brown sugar, butter and water.
Microwave in 20-second increments until brown sugar has dissolved.
Evenly divide the mixture between muffin tins.
Evenly sprinkle chopped hazelnuts in each muffin tin.
Stack two slices of sweet potato in each tin.
Drizzle with a touch of olive oil and sprinkle with salt and pepper.
Repeat until the sweet potato reach the top.
Measure milk out in a liquid measuring cup.
Whisk in cinnamon.
Evenly pour milk over each gratin.
Bake until bubbly and potatoes are cooked through, about 25-30 minutes.