Chocolate and vanilla meet twice in these light-as-a-feather souffles: dark chocolate and vanilla bean are swirled together in the batter, and the dessert is finished with cocoa powder and vanilla ice cream.
Preheat oven to 400 degrees, with rack in lower third. Butter six 6-ounce ramekins; coat with sugar. Place on a rimmed baking sheet.
Whisk together flour, 1/4 cup granulated sugar, and salt in a medium saucepan. Whisk together milk and egg yolks, then whisk mixture into saucepan along with vanilla seeds. Cook over medium-high heat until mixture comes to a boil; let boil 1 minute, whisking frequently. (Mixture should be very thick.)
Remove from heat and divide mixture evenly between two bowls. Whisk chocolate into one half; let second half stand 5 minutes, then whisk in sour cream. Press plastic wrap directly onto surface of each; let cool to warm room temperature, about 10 minutes. (Or place over an ice-water bath and whisk until cool.) Souffle bases can be made ahead to this point and refrigerated up to 1 day. Bring to room temperature and whisk to loosen before proceeding.
Whisk egg whites to break up a bit, then pour half (about 1/4 cup) into the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed until foamy; add cream of tartar and beat until soft peaks form. Gradually add 2 tablespoons granulated sugar; beat until peaks are stiff and glossy, 1 to 2 minutes. Spoon a dollop of mixture into chocolate base; whisk to lighten. Add remainder and gently fold in until combined (don't worry if some streaks remain). Repeat with other half of egg whites and vanilla base.
Divide chocolate mixture among prepared ramekins, then spoon vanilla mixture into centers. For a marbled effect, swirl with a small spatula. Bake until risen and set, about 16 minutes. Serve immediately, dusted with confectioners' sugar and cocoa powder, with scoops of ice cream.