YIELD: 16 to 20 servings
½ c. butter
½ c. firmly packed brown sugar
2 c. finely crushed vanilla wafers
2 c. finely chopped pecans
1 c. butter
1¼ c. firmly packed dark brown sugar
1 c. granulated sugar
4 large eggs
2½ c. all-purpose flour
2 tsp. baking powder
½ tsp. salt
¼ tsp. baking soda
1 container sour cream
½ c. Buttermilk
2 tsp. vanilla extract
1 semisweet chocolate chips
½ c. heavy cream
½ c. butter
6 c. powdered sugar
1 c. chocolate-covered toffee bars
For the Cake: Heat oven to 350 degrees F. In a medium bowl, stir together ½ cup melted butter and brown sugar until blended. Add vanilla wafers and pecans; toss to coat. Spread mixture in a lightly greased baking pan. Bake 15 to 20 minutes or until lightly toasted, stirring once after 10 minutes. Cool completely on a wire rack.
Beat butter at medium speed with an electric mixer until creamy. Gradually add sugars, beating well. Add egg yolks, 1 at a time, beating just until blended after each addition.
Stir together flour and next 3 ingredients in a medium bowl; in a separate bowl stir together sour cream and next 2 ingredients. Add flour mixture to butter mixture alternately with sour cream mixture, beginning and ending with flour mixture. Beat at low speed until just blended after each addition, scraping down sides of bowl as needed.
Beat egg whites in a large bowl at high speed with an electric mixer until stiff peaks form. Gently stir one-third of egg whites into batter; fold in remaining egg whites.
Grease and flour 3 (9-inch) round cake pans; line with parchment paper. Sprinkle 1 cup of pecan mixture in the bottom of each pan. Spoon batter into prepared pans. Drop pans lightly on counter to release large air bubbles.
Bake 18 to 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on wire racks. Remove from pans to wire racks; remove parchment paper. Cool completely (about 1 hour).
For the Frosting: Meanwhile, in a microwave-safe bowl, microwave chocolate chips and ¼ cup cream at medium (50% power) 2 to 3 minutes or until melted and smooth, stirring at 30 second intervals. Let cool to room temperature.
Beat softened butter at medium speed with an electric mixer until creamy. Gradually add powdered sugar and remaining ¼ cup cream, 1 tablespoon at a time, beating at low speed until blended after each addition. Beat in chocolate mixture until light and fluffy.
Spread frosting between layers and on top and sides of cake. Stir together remaining pecan mixture and finely chopped toffee bars; press on sides of cake.