Preheat oven to 180°C. Grease 2 x 20cm round cake tins.
For cake, cream butter and sugar in an electric mixer until light and fluffy. Add eggs one at a time, beating after each addition. Sift flour and baking powder three times and gently fold into wet mixture using a large spoon. Pour mixture into prepared cake tin and bake 35-40 minutes, or until springy to touch and the cake comes away from the tin. Cool for five minutes in tin and turn out onto a wire rack to cool.
For lemon curd, beat eggs in an electric mixer until pale. Strain into a bowl and place over a double boiler. Add sugar, lemon juice, butter and zest. Cook until thick and place into the freezer to cool.
For Italian meringue, place sugar and water in a saucepan and heat to 115°C using a thermometer
Whip egg whites in a clean, dry bowl of an electric mixer until soft peaks form. Once sugar syrup reaches 121°C, remove from heat and pour into whipped eggs in a slow steady stream. Continue to beat on low speed for up to seven minutes or until meringue has cooled.
To assemble, place one cake on a cake plate or board. Spread with lemon curd. Place the other cake on top. Spread meringue evenly over cakes using a pallet knife and decorate with lemon slices.