Do be careful when choosing the ground hazelnuts. Buy the best quality you can, as recent as possible and preferably organic. These will be dearer, of course, but so much better.
For an even more intense hazelnut flavour, use a hazelnut liqueur instead of the rum.
You might like to try making a hazelnut frangipane, by adding 350 g of cold
confectioner's custard (crème pâtissière, or french pastry cream) to the finished hazelnut buttercream. This should be added a little at a time with the mixer running.