Pre-heat your oven to 190°c (170°c for fan assisted ovens or Gas Mark 5) and line a couple of baking sheets with greaseproof paper. Set aside.
In a large bowl, cream together the butter/margarine and icing sugar until light and fluffy. Sift in the flour and cornflour to create a paste-like dough.
Take a small amount of your dough (about the size of a ping-pong ball) and roll into a sausage shape. Place onto your prepared baking sheets and score with a fork. Repeat with the rest of the dough, leaving space between each.Alternatively you can pipe the biscuits using a star nozzle and a cloth piping bag, if you desire. A plastic piping bag will not work however (see notes above).
Bake in your pre-heated oven for 10-12 minutes until the biscuits have spread a little, look dry and have a slight golden colour. Leave to cool on the tray.
Once your biscuits have cooled, melt your chocolate over a bain marie or in short blasts in a microwave. Dip either end of your biscuits in the chocolate, before placing back onto the grease proof paper. When all biscuits have chocolate on, pop them in the fridge to set.
While the chocolate is setting, in a large bowl mix the butter/margarine and icing sugar, until light and fluffy to create your butter cream filling. Pipe or spread a small amount of buttercream onto a biscuit, before sandwiching another biscuit on top. Repeat with the remaining biscuits and buttercream.