Whisk the egg replacer with 6 tbs of water and stand for 15 minutes.
Beat the margarine, vanilla and Natvia until light and creamy. Beat in the egg replacer until combined.
Pour in the milk and flour in batches, beating briefly in between each addition.
Pour ½ the batter into a greased and line 20cm cake tin. Sprinkle evenly with diced apple. Top with remaining batter. Smooth the top. Place the sliced of apple on top. Bake the cake for 30-35minutes, or until a skewer inserted comes out clean. Remove from the oven and stand in the tin for 10 minutes. Remove to a wire rack to cool completely.