Preheat oven to 350°F. Spray a non-stick bundt pan with baking spray or butter and flour.
Using an electric mixer, cream together the butter, shortening, vanilla, brown and granulated sugars in a large mixing bowl. Beat for 2 minutes or until creamy and light beige in color.
Add the eggs one at a time beating well after each addition stopping to scrap the sides of the bowl as needed.
In a separate bowl, use a whisk to sift together flour, baking powder and salt. Lower the speed of the mixer and add to the creamed mixture alternately with the buttermilk.
After all has been added increase the speed of the mixer to medium and beat until fully combined around 45-60 seconds.
Spread evenly in pan. Bake for 35-45 minutes or until a cake tester inserted into the center comes back clean.
Cool in the pan for 15 minutes then turn onto a cooing rack to cool completely.
Meanwhile: In a heavy bottomed saucepan over medium heat melt together brown sugar, condensed milk, butter and vanilla. Simmer for 2 minutes until sugar is completely melted. Set aside to cool to warm but, still pour-able. (It thickens as it cools)
Pour caramel frosting over cake until all has been used. May do in batches. Sprinkle with toasted walnuts or pecans, if desired.
Store in an airtight contain or chilled or at room temperature.