Preheat oven to 325 degrees and spray a 15-cup Bundt pan with baking spray with flour.Or you can coat the pan with shortening and then dust it with flour.
Whisk together the flour, baking powder, and salt in a medium bowl. Set aside.
Using an electric stand mixer with a paddle attachment, beat butter, shortening, brown sugar, and vanilla extract on medium-high speed for 3 to 5 minutes.
Beat in eggs one at a time, beating until no yellow is visible before adding the next one and stopping several times to scrape down the sides of the bowl.
With mixer on low, beat in the flour mixture in 3 additions, alternating with the buttermilk in 2 additions.
Stir in the pecans.
Transfer batter to prepared pan and bake 50 to 60 minutes or until a toothpick inserted in the center comes out clean.
Let cool in pan for 10 minutes and then invert onto a cake stand or serving plate.
Make the icing. Place butter, brown sugar, and milk in a medium saucepan. Cut the butter in pieces so it melts faster.
Bring to a boil and boil for 1 minute stirring constantly.
Remove from heat and stir in vanilla and confectioners' sugar. Continue to stir until all the lumps are out. This may take a few minutes.
Stir in pecans. Once icing starts to thicken slightly, slowly pour it over the cake. If it is still too runny, let it cool a little more. But don't let it cool too much or it will be too thick.
Garnish with pecans if desired.