Preheat the oven to 180° C (160°C Fan, Gas Mark 4). Grease and line 3 baking trays.
In a mixing bowl, beat together the butter or margarine with the sugar until creamy. Beat in the egg and Vanilla Extract.
Sift the flour and Baking Powder on top and add the Jumbo Chocolate Chips. Mix together to form a dough.
Using a teaspoon, drop 28 spoonfuls of the mixture onto the baking trays, spaced apart, and flatten each slightly. Bake in the oven for about 15 minutes until just firm and lightly golden. Allow to cool for 10 minutes then transfer to a wire rack to cool completely.
Generously spread the smooth side of 14 cookies with the Vanilla Buttercream and sandwich together with the other cookies to serve.