Preheat oven to 180°C / 350°F / Gas mark 4.
Lightly grease two deep 8inch round cake tins with butter and line with greaseproof paper.
In a large bowl add the margarine, golden caster sugar, flour, eggs and vanilla. Beat together with an electric mixer or wooden spoon until fully combined. Fold in the orange zest and chopped pecans.
Bake for 30-35 minutes until risen, golden on top and a toothpick inserted in the middle of the cake comes out clean.