Break up cakes in a large bowl until they resemble bread crumbs. Add orange zest and drench in Grand Marnier.
Line another bowl with cling wrap then push cake mixture into bowl to create dome shape. Pop into the fridge while you make the ganache.
Melt chocolate and cream together in microwave until melted , stirring every 40 seconds to avoid lumps and over cooking. When smooth allow it to cool to thicken a little - this makes it easier to spread over cake.
Decorating the cake
Melt the white chocolate in the microwave then add the desired colour. For a brighter colour result its best you work with oil based powder colourings available at good cake decorating shops.
Put the coloured chocolate into piping bag and pipe out swirls or desired patterns onto baking paper.
When the chocolate patterns have set, remove the cake from the bowl and pour the chocolate ganache all over the cake.
Peel the chocolate patterns off the baking paper and place decoratively on the cake while the ganache is still wet.