Preheat oven to 180C, 350F, Gas Mark 4
Lightly oil 2 baking sheets.
Cream the margarine, butter and icing sugar together until the mixture is light and fluffy.
Stir the chocolate until smooth, then beat into the creamed mixture. Stir in the cornflour.
Mix the baking powder with the flour, and gradually add it to the creamed mixture, beating well between additions. Beat until consistency is smooth and stiff enough for piping.
Put the mixture in a piping bag fitted with a large star nozzle.
Pipe 40 whirls onto the prepared baking sheets.
Bake for 12-15 mins or until firm to the touch.
Remove from oven and leave in trays to cool for around 2 minutes.
Using a spatula, transfer the whirls to wire racks and leave to cool.
Make the buttercream. Cream the butter with the vanilla essence until soft. Gradually beat in the icing sugar and add a little cooled boiled water, if necessary, to give a smooth consistency. When the whirls are cold, pipe or spread on the prepared buttercream, sandwich together and serve.
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