Grease and line two 9 x 1 inch round cake pans. Whisk the oil, orange zest, yoghurt, sugar and eggs in a large mixing bowl until combined. Sift the six dry ingredients together and fold into mixing bowl with a spatula until combined. Fold in carrots and walnuts.
Divide batter equally between the two pans. Bake in preheated oven at 190°C for 15 to 20 minutes, until cooked. After 5 minutes, remove cake from pan and leave to cool on the rack.
Beat the frosting ingredients until light and creamy. Spread frosting between the cake layers and decorate with marzipan carrots.