Whisk the milk and cocoa powder in a small saucepan. Bring just to a simmer over medium heat; remove from the heat and set aside.
Melt the butter in a medium saucepan over medium heat. Add the flour and cook, stirring, until mixture smells nutty and is light golden, about 1 minute. Whisk in the warm milk mixture and cook, whisking constantly, until the mixture is the consistency of a thin pudding, about 2 minutes. Remove from the heat and whisk in the egg yolks. Transfer the mixture to a large bowl and whisk in the melted chocolate until smooth. Set the base aside.
Whisk the sugar and ¼ cup water in a small saucepan. Attach a candy thermometer to the side of pan; bring teh mixture to a boil, reduce the heat to medium-high, and cook, without stirring, until thermometer registers 248 degrees F. Remove from the heat.
Meanwhile, using an electric mixer on medium-high speed, beat the egg whites, salt, and cream of tartar in a medium bowl until frothy. With the mixer running, gradually add the hot sugar mixture, drizzling down the side of the bowl. Increase the speed to high and beat the egg whites until stiff peaks form, about 4 minutes.
Fold one-third of the meringue into the base to loosen, then fold in the remaining meringue until just combined (a few streaks of meringue are fine).
Preheat the oven to 375 degrees F.
Butter the ramekins, then sprinkle with sugar, shaking out any excess; place on a rimmed baking sheet. Fill the prepared ramekins up to ¼-inch below the rim with batter. Bake, without opening oven door, until the soufflés rise and the centers are set, 18 to 22 minutes. Sprinkle the soufflés with sea salt and serve warm with chantilly cream.