Cake
2 1/2 cups flour
1 teaspoon baking soda
1/4 teaspoon salt
1 cup butter
2 cups sugar
4 eggs
1 cup buttermilk
4 1/2 ounces unsweetened chocolate
1/2 cup boiling water
2 teaspoons vanilla extract
1/4 teaspoon lemon juice
Filling
1 1/8 cups butter
3 cups powdered sugar
1 teaspoon vanilla extract
1/4 teaspoon lemon juice
2 cups superfine sugar
1 cup milk

Heat oven to 325ºF. Grease and flour four 8-inch round cake pans. Mix flour, baking soda, and salt together in a bowl. In a large bowl, cream together butter and sugar. Beat in eggs one at a time. Alternately beat in flour mixture and buttermilk till smooth. Melt chocolate. Mix chocolate, vanilla and lemon juice into batter. Divide batter evenly into prepared pans. Bake 15 minutes till a toothpick inserted in centers comes out clean. Cool 10 minutes in pans, then turn out onto wire racks to cool completely.
Caramel Filling
Mix together butter, powdered sugar, vanilla, lemon juice, superfine sugar and milk in a heavy saucepan. Stirring constantly, bring to a boil, and cook till temperature reaches 225ºF on a candy thermometer. Remove saucepan from heat and set in a large bowl filled with ice water. Beat filling with an electric mixer till spreadable. At first this seems to be just hot, hard work, but the filling will suddenly start to thicken as it begins to cool. Transfer filling to a clean bowl and it will continue to cool as you frost the top of each cake layer. Stack the frosted layers atop each other.