Preheat oven to 350°F (180°C). Grease and line three 8-inch cake pans. In a bowl, whisk flour, baking powder, baking soda, and salt. In another bowl, cream butter with both sugars until light and fluffy. Add eggs one at a time, beating well after each, then mix in vanilla. Add dry ingredients alternately with buttermilk, mixing just until smooth. Fold in toasted pecans. Divide the batter among the pans and bake for 30–35 minutes, or until a toothpick inserted comes out clean. Let cool completely.
For the caramel sauce, heat sugar in a saucepan over medium heat until melted and amber-colored. Add butter and whisk until combined. Slowly pour in cream while stirring. Let it simmer for a minute, then remove from heat and add salt. Allow to cool before using.
For the frosting, beat butter until creamy. Gradually add powdered sugar, then mix in caramel sauce and vanilla. Add milk or cream a tablespoon at a time until smooth and spreadable.
To assemble, spread caramel buttercream between each cake layer, adding a drizzle of caramel sauce and some chopped pecans in between. Frost the top and sides with the remaining buttercream. Finish with a caramel drizzle and a generous sprinkle of toasted pecans on top.
