In a mixing bowl beat together the butter and sugars until light, pale and fluffy. Add in the egg, vanilla extract and treacle and beat until well combined.
Fold through the remaining ingredients until well combined and rough dough forms. Cover the bowl with cling film and leave to rest for at least 30 minutes in the fridge.
Preheat the oven to 180˚C/350°F/Gas Mark 4 and line two baking sheets with parchment paper.
Roll out balls of the dough approximately 6cm in diameter and toss in granulated sugar before placing on the baking sheets at least 10cm apart.
Place in the oven to bake for 8 minutes or until they have spread and crack slightly. Be careful not to over bake, the edges should be firm but the interior should remain chewy.
Remove from the oven and allow to cool slightly before transferring to a wire rack to cool completely.
While the cookies cool, prepare the filling by beating the butter with the icing sugar until completely smooth.
Spread a dollop of the filling onto half the cookies with a small offset spatula and sandwich together with the remaining half of the cookies.
Source: Donal Skehan