Place the butter, sugar, lemon zest and vanilla in a stand mixer with the paddle attachment and beat for 3-4 minutes until pale and creamy. Add the grated marzipan and mix to combine. Add the eggs, one at a time, beating well after each addition. Sift in the plain flour and baking powder, then fold in the almond meal and lemon juice. Pour into prepared pan and bake for 60-65 minutes until golden and a skewer inserted into the centre comes out clean. Allow to cool in the pan slightly, then transfer to a wire rack to cool completely.