Preheat oven to 400°F. Generously butter and sugar an 8-inch ramekin.
In a saucepan, heat the brown sugar over medium high heat until it starts to melt. Once it starts to melt, stir the sugar until it melts and starts to smoke. Add the heavy cream (be careful, it will sputter as you begin). Cook the mixture on low heat until it is smooth.
In a medium sized bowl, mix the yolks, milk, cornstarch, salt, nutmeg and vanilla. Slowly add this mixture to the pan of caramel, whisking the whole time. Once you have added all the yolk mixture, return the pan to the stove and bring to a simmer over medium heat, whisking the entire time. Once it thickens continue cooking for another 2 minutes. When it is done, pour it into a bowl and put it in the freezer or over an ice bath. Stir it often so it doesn’t get too firm.
Whip the egg whites with the cream of tartar until soft peaks, then slowly add the confectioners’ sugar until glossy and stiff peaks.
Add 1/3 of the whites into the caramel pastry cream and stir to soften the pastry cream. Add another 1/3 of the whites and gently fold them in. Add the remaining whites and fold just until incorporated and immediately pour into prepared pan.
Bake for about 40 minutes or until a skewer comes out clean.
Dust with confectioners’ sugar. Serve immediately with chocolate sauce. You can pour the sauce into the center of the soufflé or serve on the plate.