To prepare cake, coat bottoms of 2 (8-inch) round cake pans with cooking spray (do not coat sides of pans); line bottoms with wax paper. Coat wax paper with cooking spray; dust with 1 tablespoon flour.
Combine boiling water and cocoa, stirring well with a whisk. Cool completely.
Place granulated sugar, 6 tablespoons butter, and vanilla in a large bowl; beat with a mixer at medium speed until well blended (about 5 minutes). Add eggs, 1 at a time, beating well after each addition. Lightly spoon 1 2/3 cups flour into dry measuring cups; level with a knife. Combine 1 2/3 cups flour, baking soda, baking powder, and salt, stirring well with a whisk. Add flour mixture and cocoa mixture alternately to sugar mixture, beginning and ending with flour mixture.
Pour batter into prepared pans; sharply tap pans once on counter to remove air bubbles. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes on a wire rack; remove from pans. Cool completely on wire rack.
To prepare frosting, melt 2 tablespoons butter in a small saucepan over medium heat. Add brown sugar and 2 tablespoons milk; cook 1 minute or until sugar melts. Remove from heat; cool slightly. Combine butter mixture and 2 teaspoons vanilla in a large bowl. Gradually add powdered sugar; beat with a mixer at medium speed until smooth. Add additional milk, 1 teaspoon at a time, beating until spreading consistency.
Place 1 cake layer on a plate; spread top with half of frosting. Place caramel dip in a small zip-top plastic bag. Snip a small hole in 1 corner of bag; drizzle half of caramel dip over frosting. Top with other cake layer. Spread remaining frosting over top of cake; drizzle with remaining caramel dip. Sprinkle with pecans.