Preheat oven to 180 degrees C. Grease and flour 3 (20cm) pans. Sift together 2 1/2 cups flour, cinnamon, allspice and nutmeg; set aside. Toss pecans in 1/2 cup flour; set aside.
In a large bowl, cream together 250g butter and 2 cups sugar until light and fluffy. Beat in the yolks one at a time. Dissolve bicarb soda in buttermilk. Beat in the flour mixture alternately with the buttermilk. Fold in the pecan mixture and the pear jam.
In a large glass or metal mixing bowl, beat egg whites until stiff peaks form. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain. Pour batter into prepared pan.
Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Let cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.
Make icing: In a large saucepan, combine 3 cups sugar and milk. Cook over low heat, stirring frequently, until sugar is dissolved. Remove from heat and set aside. Place remaining 1 1/2 cup sugar in a cast iron frypan, cook over medium heat, stirring constantly, until sugar dissolves and becomes a golden syrup. Remove from heat and carefully stir in butter. Gradually pour syrup mixture into milk mixture in saucepan. Cook over medium heat, stirring constantly, until the mixture reaches soft ball stage. Remove from heat and beat at medium speed of electric mixture for about 5 minutes or until thick enough to spread. Spread immediately on cooled cake.