Place the butter, sugar, vanilla and water in a medium saucepan over a medium heat and heat gently until the sugar dissolves, stirring occasionally. Don't let it boil.
Take off the heat and add the hazelnut flour and roughly chopped hazelnuts and mix well to combine.
Leave to cool slightly for 5 minutes.
Pour over the base and using the back of a spoon, spread out into an even layer, completely covering the base and jam.
Place in the oven and bake for 30 minutes, checking at 25 minutes to make sure the edges aren't burning, turning half way through.
Leave to cool in the tin for 15 minutes before marking into triangles using a sharp knife.
Mark into 8 cm x 7.5 cm (3.15" x 2.95") (rough guide - see below for more detail on cutting) squares, then cut each of these in half diagonally to make triangles.
Leave to cool completely in tin.
Melt the dark chocolate in a bowl set over a pan of barely simmering water or in the microwave, and then pour into a small deep bowl.
Take a nussecken triangle and dip a tip in the chocolate, hold for a second or two and then dip the other one in. Place on greaseproof paper until set completely.