Make the Cake:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
2. Cream Butter and Sugars: In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
3. Add Eggs and Vanilla: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Mix in the vanilla extract.
4. Combine Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
5. Mix Dry and Wet Ingredients: Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk. Begin and end with the dry ingredients. Mix until just combined. Fold in the chopped pecans.
6. Bake: Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then transfer to wire racks to cool completely.

Make the Butterscotch Sauce:

1. Combine Ingredients: In a saucepan over medium heat, combine the brown sugar, butter, and heavy cream. Stir continuously until the mixture comes to a boil.
2. Simmer: Allow the mixture to simmer for 3-5 minutes, stirring frequently. Remove from heat and stir in the vanilla extract and a pinch of salt. Let it cool slightly before using.

Assemble the Cake:

1. Layer the Cake: Place one cake layer on a serving plate. Drizzle with some butterscotch sauce, then top with the second layer.
2. Frost and Drizzle: Drizzle the top of the cake with more butterscotch sauce and garnish with additional chopped pecans if desired.