Preheat oven to 350°. Grease 3 8-inch round cake pans. Line bottoms with parchment paper or wax paper. Grease paper. Set aside.
Using an electric mixer on medium speed, beat butter until creamy. Gradually add brown sugar and beat well. Add eggs, one at a time, beating well after each addition.
Combine water and vinegar. Set aside.
Combine flour, baking soda, and salt. Add to butter mixture, alternating with the water/vinegar mixture. Begin and end with flour mixture. Stir in chopped hazelnuts and vanilla.
Pour one-third of the batter into one of the prepared pans. Repeat with a second pan.
Fold cocoa powder into remaining batter. Pour into third pan.
Bake until a wooden pick inserted in the center comes out clean (about 19-21 minutes). Cool in pans on wire rack for 10 minutes. Then, remove from pans, peel off paper, and allow to cool completely.
Brush Frangelico over cake layers.
Melt chocolate and butter in a heavy saucepan over medium-low heat. Allow to cool for about 10 minutes. Stir in pecans and vanilla. Cool mixture, stirring occasionally, until it is consistent for spreading (about 45 minutes).
Place one white cake layer on serving plate. Spread a third of topping on top of cake. Do not cover sides. Top with the chocolate cake layer. Spread a third of topping on top of cake. Top with remaining white cake layer. Spread remaining topping on top of cake. Garnish, if desired, with toasted hazelnuts. Allow cake to set several hours before serving.