Beat the butter and the icing sugar together until really soft and creamy.
200 g unsalted butter, 75 g icing sugar
Beat in the cocoa, salt and vanilla extract until well combined.
25 g cocoa powder, ½ tsp vanilla extract, pinch salt
Sieve in the flour and beat until smooth.
180 g plain flour (all purpose flour)
Add the milk, if needed and beat again until smooth. The dough needs to be fairly stiff, but pipeable.
1 tbsp milk
Spoon into a piping bag fitted with a large star nozzle and pipe 24 x 5cm diameter whirls spaced well apart on a lined baking sheet.
Place in the fridge to set for 20 minutes or so.
Bake for 10 to 12 minutes in the centre of the oven at 180℃ (160℃ fan, 350℉, Gas 4). The biscuits should be just firm to the touch.
Leave for a couple of minutes then carefully transfer to a wire rack to cool completely
Vanilla Buttercream
Beat the butter and icing sugar until very pale and creamy, then beat in the vanilla paste or extract.
50 g unsalted butter, 100 g icing sugar, ½ tsp vanilla bean paste
Spread the buttercream onto 12 of the biscuits and the jam on the other 12, then sandwich together.
12 tsp jam of your choice
Notes
If the whirls are not placed in the fridge, they are more likely to spread and lose their definition.Please note: calories and other nutritional information are per serving. They're approximate and will depend on exact ingredients used.