Cake: Preheat oven to 180 degrees C. Generously butter 3 20cm cake tins.
Divide the 2 cups chopped pecans evenly between the cake tins. Shake to coat bottoms and sides of tins.
In a large bowl, cream together the butter, copha and sugar until light and fluffy. Beat in the yolks one at a time, then stir in the vanilla. Combine the flour and bicarb soda and beat into creamed mixture alternately with the buttermilk. Stir in 1 cup chopped pecans.
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. Divide batter equally into prepared cake tins.
Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Cool in their cake tins on wire racks 10 minutes. Invert layers onto wire racks lined with greaseproof paper. Brush tops and sides of cake layers with 2/3 cup golden syrup and cool completely.
Filling: In a large saucepan, combine brown sugar and cornflour. Stir in 4 egg yolks, cream, golden syrup and salt. Bring mixture to a boil over medium heat. Continue boiling, whisking constantly for 1 minute, or until thickened. Remove from heat. Whisk in butter and vanilla. Chill.
To Assemble: Place first layer on serving plate, pecan side up. Spread with half of the filling. Place second layer, pecan side up, on filling. Spread with remaining filling. Top with remaining cake layer, pecan side up.