Pre-heat the oven to 180°c and grease and line two 21cm diameter, 10cm deep cake pans with baking paper.
Sift together the flour, baking powder and salt and set aside.
IN a separate bowl, combine the remaining ingredients and mix well.
Pour the egg and oil mixture into the dry ingredients and mix to combine.
Pour the batter into the prepared cake pans and bake for 30-45 minutes or until a skewer inserted comes out clean.
Remove from the oven and allow to cool before turning out onto a wire rack. Carefully half each cake horizontally and set aside.
To make the buttercream, whip the butter and icing sugar together for 5-7 minutes until soft, fluffy and very pale. With the mixer running, pour in the cooled melted chocolate. When the chocolate has been incorporated, add the lemon juice and vanilla and mix through. If the frosting is a little soft, place in the fridge to firm up for 5-10 minutes.
To make the filling, whip together the sugar and cream until stiff peaks form.
To assemble the cake, sandwich the cake layers together with the whipped cream then generously cover in the white chocolate frosting.