1. Preheat oven to 325°.
2. Heat milks over medium-high heat in a heavy saucepan to 180° (do not boil). Combine brown sugar and next 5 ingredients (through egg yolks) in a medium bowl; stir well with a whisk. Gradually pour 1/2 cup hot milk mixture into egg mixture, whisking constantly. Return egg mixture to pan, stirring constantly. Strain mixture through a fine sieve into a bowl. Stir in butter and vanilla. Cover and chill 1 hour.
3. Divide mixture evenly among 6 (8-ounce) custard cups. Place cups in a 13 x 9-inch metal baking pan; add enough hot water to come halfway up sides of cups. Bake at 325° for 24 minutes or until center barely moves when cup is touched. Remove cups from pan; cool on a rack 20 minutes. Cover and chill at least 1 hour.
4. Place cream and powdered sugar in a bowl; beat with a mixer at high speed until stiff peaks form. Top custards with cream.