Preheat oven to 350 degrees F (175 degrees C). Grease and flour 2 (9 inch) pans. Sift together the flour, baking powder, baking soda, salt, cinnamon, 3/4 teaspoon nutmeg, cloves and allspice. Set aside.
In a large bowl, cream together the shortening, brown sugar and 1 cup white sugar until light and fluffy. Beat in the eggs one at a time, then stir in 2 teaspoons vanilla. Beat in the flour mixture alternately with 1 1/2 buttermilk, mixing just until incorporated. Stir in pecans. Pour batter into prepared pans.
Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Let cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.
To make the frosting: In a large bowl, combine butter, 1/2 teaspoon salt, 1 teaspoon vanilla and about 1 cup of the confectioners' sugar. Beat well, then gradually beat in the remaining confectioners' sugar alternately with 1/4 cup buttermilk. Beat until smooth and creamy and spread on cake.