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Heat the oven to 180C/160C fan/ gas 4. Butter two 20cm loose-bottomed cake tins and line with baking parchment. Beat the butter and caster sugar together in a large bowl using an electric whisk until pale and fluffy. Add the eggs, one at a time, until combined. Tip in the vanilla, flour and baking powder and fold until smooth, then gently fold in the orange zest and blackberries, being careful not to overmix.
Divide the batter between the two tins and bake for 35-40 mins until golden and a skewer inserted into the middles comes out clean. Cover with foil after 30 mins if they start to colour too quickly.
Meanwhile, make the drizzle. Put the orange juice, zest and sugar in a small pan and simmer for 4-5 mins until a thin syrup forms. Pour into a heatproof bowl and set aside.
When the sponges are ready, drizzle the syrup over both while still warm and brush over evenly using a pastry brush. Leave to cool completely in the tins.
Meanwhile, make the buttercream for the topping. Beat the butter and icing sugar in a large bowl with an electric whisk until light, pale and fluffy. Whisk in the orange zest and milk until loosened slightly. Spoon into a piping bag fitted with a large round nozzle. Beat the marmalade to loosen, then transfer a little to a piping bag fitted with a small round nozzle.
Remove the cooled sponges from the tins, and place one on a cake board or plate. Pipe buttercream dots on top in a circle around the edge, then three dots in the middle. Spoon some marmalade into the gaps between the buttercream, then top with the second sponge. Pipe more buttercream dots all over the surface, then fill in the gaps by piping in the remaining marmalade. To finish, scatter with the blackberries and extra orange zest.