Prepare the Cake Batter: Cream softened butter and granulated sugar until light and fluffy. Add eggs one at a time, followed by melted butterscotch chips and vanilla extract. In a separate bowl, whisk flour, baking powder, baking soda, and salt. Gradually mix in dry ingredients and buttermilk, beginning and ending with dry ingredients.

Bake the Cake Layers: Divide the batter between two greased 9-inch round cake pans. Bake at 350°F (175°C) for 25-30 minutes or until a toothpick comes out clean. Cool in pans for 10 minutes, then transfer to wire racks to cool completely.

Prepare the Butterscotch Frosting: Beat softened butter until creamy. Gradually add powdered sugar, alternating with butterscotch sauce and heavy cream, beating until light and fluffy. Stir in vanilla extract.

Make the Crunch Topping: Mix chopped pecans, crushed pretzels, brown sugar, and melted butter. Spread on a baking sheet and bake at 350°F (175°C) for 8-10 minutes or until golden. Let cool.

Assemble the Cake: Place one cake layer on a plate, spreading frosting on top. Add the second layer, then frost the top and sides. Sprinkle cooled crunch topping and press lightly to adhere.

Chill and Serve: Refrigerate for at least 30 minutes before slicing and serving.