Step 1
Preheat oven to 375 degrees F (190 degrees C). Line a rimmed baking sheet with parchment paper.
Step 2
Brush bottom and sides of 2 (5-ounce) ramekins lightly with 1 teaspoon melted butter; cover bottom and sides right up to the rim. Add 1 tablespoon white sugar to ramekins. Rotate ramekins until sugar coats all surfaces. Pour off extra sugar.
Step 3
Place chocolate pieces in a metal mixing bowl. Place bowl over a pan of about 3 cups hot water over low heat. Do not let water boil or come to a simmer.
Step 4
Melt 1 tablespoon butter in a skillet over medium heat. Sprinkle in flour. Whisk until flour is incorporated into butter and mixture thickens about 1 minute. Reduce heat to low. Whisk in cold milk until mixture becomes smooth and thickens 2 or 3 minutes. Remove skillet from heat. Transfer mixture to bowl with melted chocolate. Add salt and very small pinch of cayenne pepper. Mix together thoroughly. Add egg yolk and mix to combine. Leave bowl above the hot (not simmering) water to keep chocolate warm while you whip the egg whites.
Step 5
Place 2 egg whites in a mixing bowl; add cream of tartar. Whisk until mixture begins to thicken and a drizzle from the whisk stays on the surface about 1 second before disappearing into the mix 2 or 3 minutes. Add 1/3 of sugar and whisk in. Whisk in a bit more sugar about 15 seconds; whisk in the rest of the sugar. Continue whisking until mixture is about as thick as shaving cream and holds soft peaks 3 to 5 minutes.
Step 6
Transfer a little less than half of egg whites to chocolate. Mix until egg whites are thoroughly incorporated into the chocolate 1 or 2 minutes. Add the rest of the egg whites; gently fold into the chocolate with a spatula lifting from the bottom and folding over. Stop mixing after the egg white disappears. Divide mixture between 2 prepared ramekins. Place ramekins on prepared baking sheet.
Step 7
Bake in preheated oven until scuffles are puffed and have risen above the top of the rims 12 to 15 minutes.