Step 1

Place sweet potatoes on a microwave-safe plate. Prick sweet potatoes with the tip of a knife. Microwave until very soft, about 10 minutes. Let cool briefly until safe to handle.

Step 2

Preheat the oven to 425 degrees F (220 degrees C).

Step 3

Halve sweet potatoes and scoop flesh out into a bowl.

Step 4

Separate egg whites and yolks into individual mixing bowls. Generously butter four 5-ounce ramekins.

Step 5

Transfer 1 cup mashed sweet potato to the bowl of egg yolks. Pour in milk. Season with salt and cayenne. Whisk until smooth, slowly at first and faster once mixture starts to come together.

Step 6

Add a pinch of salt to the egg whites. Beat with a clean whisk until peaks are well-defined but still soft. Transfer 1/2 of the egg whites to the sweet potato mixture. Fold in with a spatula somewhat gently until most of the white streaks disappear. Add the remaining egg whites and fold in very gently, turning the bowl and scraping the sides, until whites are mostly blended in; a few streaks should still be visible.

Step 7

Gently spoon sweet potato mixture into the ramekins.

Step 8

Bake in the preheated oven until fully inflated and browned, about 15 minutes. Garnish with sage. Unmold or serve in ramekins.