1.Preheat the oven to 180°C (160°C fan-forced). Grease 18 muffin pans (1/3-cup/80ml capacity).
2.Beat the butter and sugar in a small bowl with an electric mixer until light and fluffy. Add egg yolks one at a time, beating well between additions. Transfer mixture to a large bowl, stir in cooled chocolate, then fold in hazelnuts, liqueur and vanilla extract.
3.Beat egg whites in a clean medium bowl with an electric mixer until soft peaks form. Fold egg white into the chocolate mixture in two batches.
4.Divide mixture among prepared pans. Bake 25 minutes or until just cooked. Stand cakes in pans for 5 minutes before turning onto wire rack; cool cakes upside-down.
5.To make chocolate ganache, combine chocolate and butter in a heatproof bowl over a pan of barely simmering water, stirring occasionally, until smooth and glossy. Remove from heat, stand for about 30 minutes or until thickened slightly.
6.Pour a little chocolate ganache over each cake; decorate tops with chocolates. Stand at room temperature until set.
Un-iced cakes can be made four days ahead; store covered in an airtight container at room temperature or suitable to freeze. Ice cakes on the day of serving. Chocolate and butter suitable to microwave.Note