1. Preheat oven to 325ºF.
2. In a large mixing bowl, sift together flour, baking powder and salt. Add brown sugar and mix well. Add oil, egg yolks, cold water and vanilla to the mixture and beat until smooth.
3. Beat egg whites with cream of tartar until stiff peaks form. Combine egg yolk mixture and egg whites and fold together gently.
4. Pour batter into an ungreased 10-inch tube pan and bake for 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.
5. To make penuche frosting: Combine all ingredients in a saucepan and cook, stirring constantly, over low heat until combined. Once combined, bring to a full boil, stirring constantly. Boil 7 minutes, then remove from heat and let cool completely. Chill 2 hours and beat until thick and creamy. Spread over sides and top of cooled cake.