Preheat the oven to 180°C, gas mark 4. Grease and line a 1.5-litre loaf tin.
Put the butter and 180g sugar in a bowl. Beat with an electric hand whisk until pale and fluffy.
One at a time, beat in the eggs. Stir in the flour, ground almonds, baking powder, 2 tbsp lemon curd and lemon zest.
Spoon the batter into the prepared tin and level the surface with a spoon. Bake for 40-45 minutes, covering with foil after 25 minutes, until a skewer inserted into the centre comes out clean.
Combine the lemon juice and remaining caster sugar. Use a skewer to pierce holes right through the warm loaf then spoon the drizzle over the top. Leave to cool in the tin.
When the loaf is completely cold, carefully slice horizontally into 3 even layers.
In a bowl, beat the icing sugar, cream cheese, mascarpone and remaining lemon curd until just thick enough to hold its shape.
Transfer the icing to a piping bag and cut a 1cm hole at the tip.
Place the bottom layer of the loaf crust-side down on a cake stand or plate. Pipe peaks of icing from the edge to the centre of the loaf. Carefully top with the middle layer of sponge then repeat the icing process. Finish with the top layer.