Start by making the frosting: Melt 6 tablespoons butter in a deep, heavy-duty saucepan over low. Continue to cook, slowly, until the butter stops splattering and the milk solids have browned, about 5 minutes. Remove pan from heat, and stir in brown sugar, cream, syrup, salt, and rum.
Return pan to medium heat and cook, stirring occasionally, until the sugar dissolves. Increase heat to medium-high, and bring to a boil. Boil, stirring constantly, for 2 minutes. Transfer the mixture the mixture to the bowl of a stand mixer, fitted with the paddle attachment. Beat on low speed until it is lukewarm, about 8 to 10 minutes. Gradually add the powdered sugar and vanilla, and beat on low until combined after each addition. Beat until frosting is completely cool and consistency is thick, creamy and spreadable, 6 to 10 minutes. Add remaining 2 tablespoons and beat on medium speed until fully incorporated. Cover frosting until ready to use.
To make the cakes, preheat oven to 350 degrees F. Grease and flour two jumbo muffin pans, set aside.
In a large bowl, combine the flour, baking powder, and salt. Set aside.
Beat together the sugar and butter until light and creamy, about 4 to 5 minutes. Add the eggs one at a time, beating well after each addition. Stir in the vanilla. Add the dry ingredients, alternately with the buttermilk, starting and ending with the dry ingredients. Make sure to blend until just combined after each addition. You don't want to over-mix at this point. Fold in the chopped pecans and toffee bits. Divide batter among the prepared pans. Bake until a toothpick inserted in the middle comes out clean, about 25 to 30 minutes. Cool in pans on a wire rack for 10 minutes. Invert the cakes onto wire rack and let cool completely.
To decorate the cakes, frost the sides of each and roll around in the chopped pecans and toffee bits. Then frost the tops with a knife or a piping bag and decorative tip. Serve immediately or cover in plastic and store in fridge for up to 3 days. Let come to room temperature before eating. Enjoy!