Preheat oven to 180 °C (160 °C fan). Grease and line two 8‑inch (20 cm) round cake tins.

In a mixing bowl, cream together softened butter, caster sugar, lemon zest, and vanilla until pale and fluffy.

Beat in eggs one at a time until well incorporated.

Sift flour, baking powder, and salt together, then fold into the batter until just combined.

Divide batter evenly between prepared tins. Bake for 20–22 minutes, or until a skewer inserted comes out clean. Allow cakes to cool completely on a wire rack.

Meanwhile, make lemon curd: whisk together yolks, sugar, lemon juice, and zest in a heatproof bowl. Place over a simmering pan of water (bain‑marie) and stir constantly until thickened (~8–10 minutes). Remove from heat and stir in butter cubes until smooth. Chill until thick.

Prepare buttercream: beat softened butter until creamy, then gradually add icing sugar, lemon juice, and zest until smooth and spreadable.

Assemble cake: place one sponge layer on a cake board or plate. Spread with a generous layer of cooled lemon curd. Add the second sponge. Pipe or spread buttercream on top and sides of the cake.

Decorate with extra lemon zest or thin lemon slices if desired. Chill briefly to set, then slice and serve.