To make the sponge mixture, combine the butter, icing sugar and salt in a bowl. Add the eggs and continue to whisk until the mixture becomes lighter in colour. Stir in the orange zest.
Grind the walnuts, add to the flour and stir into the mixture. Line the base of a spring form pan (approx. 26 cm in diameter) with baking paper, grease the rim. Transfer 2/3 of the mixture to the pan and smooth down. Stir the jam until smooth, spread over the cake base leaving a gap of approx. 2 cm all the way around. Transfer the remaining mixture to a piping bag with a serrated nozzle (approx. 14 mm in diameter) and pipe around the edge.
Bake for approx. 35 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool a little, remove the tin frame. Sprinkle with roughly chopped walnuts before serving.