How to Make It
Prepare the Frosting: Melt 3/4 cup of the butter in a deep, heavy-duty saucepan over low. Increase heat to medium-high, and bring butter to a boil, stirring constantly. Cook, stirring constantly, until butter is fragrant and milk solids start to brown, about 5 minutes.
Remove pan from heat, and stir in brown sugar, cream, syrup, salt, and, if desired, bourbon. Return pan to medium heat, and cook, stirring occasionally, until sugar dissolves. Increase heat to medium-high, and bring to a boil. Boil, stirring constantly, exactly 2 minutes. Remove mixture from heat, and pour into bowl of a heavy-duty electric stand mixer fitted with paddle attachment. Beat on low speed until mixture is lukewarm, about 8 to 10 minutes. Gradually add powdered sugar and vanilla, and beat on low speed until combined after each addition. Beat until frosting is completely cool and consistency is thick, creamy, and spreadable, 6 to 10 minutes. Add remaining 4 tablespoons butter, 1 tablespoon at a time, and beat on medium speed until fully incorporated after each addition. Cover frosting, and let stand at room temperature until ready to use.
Prepare the Cake: Preheat oven to 350°F. Lightly coat 2 (9-inch-round x 2-inch-deep) cake pans with cooking spray, and line bottoms with parchment paper. Sift together flour, baking powder, and salt in a medium bowl. Set aside.
Beat sugar and butter in a large bowl with an electric mixer on medium-high speed until thick and creamy, 4 to 5 minutes. Add eggs, 1 at a time, and beat on medium speed until well blended after each addition. Beat in vanilla. Add flour mixture, one-third at a time, to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat on low just until smooth after each addition. (Do not overbeat or the cake will be tough.) Fold in chopped pecans and toffee bits. Divide batter evenly between cake pans, smoothing surface.
Bake in preheated oven until a wooden pick or cake tester inserted into center of cake comes out clean, 30 to 35 minutes. Cool in pans on a wire rack 10 minutes. Invert layers onto wire rack, and peel off parchment paper. Cool completely, about 1 hour.
Place 1 cake layer on a serving plate; spread evenly with 1 heaping cup of frosting. Chill 15 minutes. Top with remaining cake layer, and frost top and sides of cake with remaining frosting. Sprinkle pecan halves in a ring around top edge of cake. If desired, crush sea salt with your fingers, and sprinkle over pecans. Serve immediately, or store, covered, at room temperature for 3 to 5 days.
Make the frosting first to give it a chance to firm up as it cools while you prepare and bake the cake layers.