Preheat the oven to 180°C (160°C fan). Line two baking trays with parchment paper.
In a large mixing bowl, cream together the softened butter and icing sugar until light and fluffy.
Sift in the plain flour and cornflour, then add the vanilla extract. Mix until a smooth dough forms.
Transfer the dough to a piping bag fitted with a large star nozzle.
Pipe small whirl shapes onto the prepared baking trays, leaving space between each whirl.
Bake in the preheated oven for 12-15 minutes, or until the whirls are golden brown.
Remove from the oven and allow to cool on the trays for 5 minutes before transferring to a wire rack to cool completely.
Once the whirls are cool, sandwich them together with raspberry jam.
Melt the dark chocolate in a heatproof bowl set over a pan of simmering water, or in the microwave in short bursts, stirring frequently.
Dip the Viennese whirls halfway into the melted chocolate and place on a parchment-lined tray to set.
