Preheat the oven to 180 degrees Celsius. Grease a 20cm round cake pan and line the base and side with baking paper. Use an electric mixer to beat the butter and sugar in a bowl until pale and creamy. Add the eggs one at a time, beating well after each addition. Add in the lemon rind and stir. Add the plain flour and the self-raising flour and stir. Next, add the sour cream and stir.....and the lemon juice and stir. Spoon into the prepared pan and smooth the surface. Bake for 45 minutes or until a skewer inserted in the centre comes out clean. Turn onto a wire rack to cool completely. For the lemon curd, combine the butter, sugar, lemon juice, lemon rind and egg in a saucepan over low heat. Cook for five minutes, stirring constantly or until the mixture coats the back of a spoon. Strain through a fine sieve into a clean bowl. Cover with plastic wrap and place in the fridge to chill. To make the meringues, preheat the oven to 100 degrees Celsius. Use an electric mixer to whisk eggwhites in a clean, dry bowl until soft peaks form. Gradually add the sugar one tablespoon at a time, whisking well after each addition. Continue whisking until the sugar is completely dissolved. Place into a piping bag fitted with a 1cm fluted nozzle. Pipe the meringues onto the lined tray. Bake for 1 hour or until dry to the touch. Turn the oven off. Leave the meringues in the oven with the door ajar for one hour to cool completely. Use an electric mixer to whisk the mascarpone, cream and half the lemon curd in a bowl until soft peaks form. Use a large serrated knife to split the cake into two. Place one cake layer in the base of a large serving dish. Drizzle with half the liqueur if you're using it and then spread with half the blackberry jam. Then spoon over half the mascarpone mixture and half the remaining lemon curd. Sprinkle with half the blackberries and meringues. Continue layering with the remaining cake, liqueur, jam, mascarpone mixture and lemon curd.....finishing off with the blackberries and the meringues on top.