Preheat the oven to 320°F/160°C.
Use a pastry brush to brush the insides of four 3.5 x 2.2-inch (9x5.5cm) ramekins with the softened unsalted butter, ensuring to stroke upward with the brush.
Sprinkle a small amount of sugar in the ramekins and swirl the sugar around to coat the ramekins. Tap the ramekins to remove any excess sugar. Set aside.
To make the beurre manié, in a medium bowl, combine the butter, flour, and salt. Use a spatula to beat until a thick, dry paste forms. Set aside.
To make the soufflé base, in a medium saucepan on the stovetop over medium heat, combine the milk and sugar. Bring the mixture to a gentle simmer.
Remove the saucepan from the heat and whisk in the beurre manié. Keep whisking until the beurre manié has broken down and been absorbed into the milk. About 30 seconds.
Return the saucepan to the heat and whisk for 1-2 minutes until the mixture has thickened. When you lift the mixture with the whisk, it should be slightly elastic in consistency and not too runny.
Remove the saucepan from the heat and whisk in the chopped dark chocolate until completely melted.
Transfer the mixture to a large bowl and add the egg yolks. Beat with a spatula until the eggs have been completely combined. Set aside.
To make the French meringue, in a medium bowl, combine the egg whites and lemon juice. Whisk with an electric hand whisk on a medium-low speed until frothy, then slowly add the sugar 1 tablespoon at a time.
Increase the speed of the hand whisk to medium-high and whisk until the meringue has stiff peaks.
Fold the meringue into the soufflé base in three stages, ensuring you scoop the bottom of the bowl and there are no streaks of meringue left.
Transfer the mixture to a piping bag fitted with a large round tip nozzle.
Use the piping bag to fill the ramekins with the mixture. Use an offset spatula to completely smooth the tops so the mixture is flush with the rims of the ramekins. Use your thumb to carefully create a very small lip around the inside edge of each ramekin. Clean the outsides of the ramekins of any of the mixture.
Place the ramekins in the oven and bake for about 16 to 20 minutes. They should be well risen above the rim.
Remove the ramekins from the oven and serve immediately.