Chop the Hershey bars and place in a microwave safe bowl. Microwave at 20 second intervals, stirring after each one, until melted. Set aside.
Preheat oven to 325 degrees.
Using electric mixer with paddle attachment, cream butter and sugar until light and fluffy. This can take up to 5 minutes.
Beat in eggs one at a time, stopping to scrape down the sides of the bowl several times.
Whisk together flour, baking soda, and salt in a medium bowl.With mixer on low, add flour mixture, alternating with buttermilk, adding flour in 3 additions and buttermilk in two.
Add the melted chocolate, vanilla, and pecans and mix on low just until combined.
Spray a Bundt pan with baking spray with flour.Transfer batter to bundt pan. Bake for 1 hour 20 minutes to 1 hour 30 minutes.
Let cool in pan 15 minutes and then invert onto a serving platter.