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Heat oven to 180C/160C fan/gas 4. Butter and line a 20 x 24cm cake tin with baking parchment. Cream the margarine or butter with the caster sugar using an electric whisk until light and fluffy, then beat in the eggs one at a time. Sieve in the flour and fold it in. Add the orange zest and juice, fold that in as well, then pour into the prepared tin, smoothing over the top. Bake for 25 mins until golden. A skewer should come out clean when inserted in the middle.
Remove the cake from the oven, leave for 5 mins, then remove from the tin, peel away the parchment and leave to cool on a wire rack.
Meanwhile, make the topping by mixing the mascarpone, icing sugar, orange juice and vanilla until smooth. Cut open the passion fruit, scoop out the seeds, then add half to the mix, reserving the rest.
Use a palette knife to spread the topping over the cooled cake, then drizzle over the remaining passion fruit seeds.