Preheat the oven to 180°C/160°C fan/Gas Mark 4. Line two baking sheets with nonstick baking paper.
Place 100g of butter and sugar in a bowl and using an electric mixer beat until light and fluffy. Add the egg yolks one at a time beating well after each addition then beat in the vanilla.
Sift over the flour and gently fold into the mixture until just combined.
Spoon into a piping bag fitted with a star nozzle and pipe tight waves in 7cm lengths onto the lined baking sheets. Transfer to the oven to bake for 12 minutes until lightly golden. Cool slightly before transferring to a wire wrap to cool completely.
Place the remaining butter in a bowl and add the chocolate. Gently melt in the microwave then stir to combine. Lay a sheet of nonstick baking paper onto a large chopping board. Dip each cooled biscuit half way into the chocolate then place on the prepared paper to set.
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