Instructions:
- Roast sweet potatoes at 400°F (200°C) until very soft. Scoop out flesh, mash/purée until smooth, and measure out 1 cup.
- Thoroughly brush the inside of 4 x 6-ounce ramekins with softened butter, using upward strokes. Chill briefly. Coat the buttered surface completely with ¼ cup granulated sugar, tapping out the excess, and chill again.
- Make the Roux: Melt 3 Tbsp butter in a saucepan over medium heat. Whisk in the flour and cook for 1 minute until nutty smelling.
- Create the Panade: Gradually whisk in the warm milk until smooth. Cook, stirring constantly, until the mixture thickens like thick wallpaper paste. Remove from heat.
- Enrich the Base: Whisk the ¼ cup sugar and egg yolks together. Temper the yolks by slowly drizzling a spoonful of the hot panade mixture into them while whisking vigorously, then pour the tempered yolks back into the saucepan mixture.
- Combine Flavours: Stir in the 1 cup sweet potato purée, spices (cinnamon, nutmeg, cloves), and vanilla extract until perfectly homogenous. Transfer the base to a large bowl and let it cool slightly.
- Whip Egg Whites: In a clean, grease-free bowl, whisk the 4 egg whites (and cream of tartar, if using) until soft peaks form.
- Form Stiff Peaks: Gradually add the final ¼ cup of sugar, continuing to whip until glossy, stiff peaks form that stand straight up without drooping.
- Lighten the Base: Gently fold about one-third of the meringue into the cooled sweet potato base to loosen it up.
- Gentle Folding: Carefully transfer the remaining meringue into the lightened base. Use a rubber spatula to gently fold, cutting down the middle and sweeping up the sides, until just combined. Stop folding the moment no white streaks remain.
- Fill and Smooth: Carefully spoon the mixture into the prepared ramekins, filling them nearly to the top. Run your thumb around the top edge of each ramekin to create a slight channel for a straight rise.
- Bake: Place ramekins on a baking sheet. Bake in a preheated oven at 375°F (190°C) for 25–30 minutes. Do not open the oven door for the first 20 minutes. Serve immediately when puffed and golden brown.