Pre-heat oven: 190°C / 170°C (fan) / Gas Mark 5 / 375°F
Cream the butter until soft, then add the remaining biscuit ingredients and mix together until a smooth and thick dough forms.
Roll the dough into walnut-sized balls and gently flatten and shape them into discs in your palm. If your dough feels too sticky, refrigerate the mix for 5-10mins before trying again.
Place the discs onto a lined baking sheet, leave a few cm gap between each. Use the back of a fork to press a small indent into each disc, before baking for 8-12mins until lightly golden at the edges.
Once the biscuits have cooled, break the chocolate into pieces and set aside ⅓ for later. Slowly melt the ⅔ of chocolate in a heatproof bowl either in the microwave (15secs at a time, stirring between) or over a boiling saucepan of water.
Remove from the heat and add in the remaining ⅓ of chocolate and stir until fully melted.
For each biscuit, dip one side into the melted chocolate, allowing the chocolate to drip off into the bowl before setting it down on the baking paper to set for roughly 20-30mins.
Once the chocolate has set and is hard to touch, turn a biscuit over and add a large spoonful of the buttercream into the centre.
Sandwich another biscuit on top, gently pressing together until the buttercream reaches the edges of the biscuits. Repeat until all of the biscuits have been filled and sandwiched.