First make the biscuits.
Preheat the oven to 190C/375F/Gas 5.
Line 2 baking trays with parchment paper. Fit a piping bag with a large star nozzle.
Put the butter and icing sugar into a bowl and beat well until pale and fluffy. Add the flour and baking powder and beat well again until thoroughly mixed.
Spoon the mix into the piping bag and pipe out rounds onto the parchment paper, spacing them well apart. Bake in the oven for 10-15 minutes until pale golden.
Cool on a wire rack.
While they are cooling make the filling. With an electric mixer whizz the butter and the icing sugar together until smooth. Add the mascarpone and mix until smooth.
Spread the filling onto the base of one biscuit and spread the blackcurrant jam onto the base of a second biscuit and sandwich them together. Repeat until all of the filling is used up.