Souffles are often made with dark chocolate -- but Martha loves using excellent-quality milk chocolate instead. It gives the dessert a creaminess and flavor that are almost reminiscent of hot chocolate.
Preheat oven to 400 degrees with rack in lower third. Butter six 6-ounce ramekins; coat with sugar. Place on a rimmed baking sheet.
Heat chocolate in small heatproof bowl set over a pan of simmering water until melted, then stir until smooth. Remove from heat and keep warm.
Bring milk, vanilla bean and seeds, and salt to a simmer in a medium saucepan over medium-low. In a large bowl, whisk together yolks and 1/4 cup sugar until pale and fluffy, 1 to 2 minutes. Add flour and whisk until well combined.
Remove vanilla bean from milk mixture (discard or reserve for another use), then gradually add half to yolk mixture, whisking constantly. Whisk yolk mixture into remaining milk mixture in saucepan. Bring just to a boil, whisking constantly, then reduce heat and simmer until thick and smooth, 1 to 2 minutes. Whisk in melted chocolate. Transfer to a large bowl and cover with plastic wrap, pressing it directly onto surface of mixture. Let cool completely.
In another bowl, whisk egg whites and cream of tartar until soft peaks form. Gradually add remaining 2 tablespoons sugar and whisk until peaks are stiff and glossy, 1 to 2 minutes.
Spoon one-quarter of whites mixture into custard base, then whisk thoroughly until smooth. Gently fold in remaining whites mixture until combined. (Don't worry if some streaks remain.) Divide evenly among prepared ramekins. Bake until risen and set, 16 to 18 minutes. Serve immediately, with sweetened whipped cream.