Whisk together flour, baking powder and salt. Add brown sugar; mix well. Add oil, egg yolks, cold water and vanilla; beat until smooth.
Beat egg whites with cream of tartar until stiff peaks form. Gently fold into batter.
Pour batter into ungreased 10-inch tube pan; bake 55 minutes or until the top springs back when lightly touched. Invert cake and let it hang upside down on a bottle until completely cooled.
Loosen sides with a knife or spatula, then turn pan over and knock pan to release cake.
To make penuche frosting, combine all ingredients in a saucepan; cook, stirring constantly, over low heat until combined, then bring to a full boil. Boil 7 minutes; remove from heat. Let cool completely. Chill 2 hours. Beat until thick and creamy. Spread over sides and top of cooled cake.