How to Make It
To prepare cake, preheat oven to 350°.
Place the first 3 ingredients in a large bowl, and beat with a mixer at medium speed until light and fluffy (about 4 minutes). Add eggs, 1 at a time, beating well after each addition. Beat in 1 teaspoon vanilla. Weigh or lightly spoon flour into dry measuring cups, and level with a knife. Combine flour and the next 4 ingredients (through 1/4 teaspoon salt) in a bowl, stirring well with a whisk. Add the flour mixture and buttermilk alternately to sugar mixture, beginning and ending with the flour mixture, and beat just until incorporated.
Divide the batter evenly between 2 (8-inch) round metal cake pans coated with baking spray. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Cool in pans for 10 minutes on a wire rack. Remove from pans, and cool completely on wire rack.
To prepare mousse, place 1/4 cup granulated sugar in a medium, heavy saucepan over medium-high heat, and cook until sugar dissolves, stirring gently as needed to dissolve sugar evenly (about 4 minutes). Carefully stir in milk and 1 teaspoon melted butter (mixture will bubble and caramelized sugar will harden and stick to spoon); cook 1 minute or until sugar melts. Combine cornstarch and 1 tablespoon water in a small bowl. Add cornstarch mixture to pan; bring to a boil. Cook until thick and bubbly (about 1 minute), stirring constantly. Remove from heat; stir in bourbon, 1/4 teaspoon vanilla, and dash of salt. Pour into a bowl; cover and chill for 20 minutes. Uncover and fold in one-third of whipped topping. Gently fold in remaining whipped topping.
Place 1 cake layer on a plate. Spread mousse over cake, leaving a 1-inch border around outside edge. Top with remaining cake layer.
To prepare topping, place brown sugar and the next 5 ingredients (through dash of salt) in a small, heavy saucepan over medium heat, and bring mixture to a boil, stirring frequently. Cook until a candy thermometer registers 234° (about 3 minutes), stirring frequently. Remove from heat; stir in chopped pecans and 1/4 teaspoon vanilla. Drizzle over top of cake, spreading it out over edges. Let stand until topping is set.