• Add the room temperature unsalted butter and caster sugar to a bowl and mix until just combined and creamy.
• Add the vanilla, plain flour and cocoa powder and start to beat together, adding the milk a little at a time until the dough clumps together. You’re aiming for a dough that just holds together and isn’t sticky, so try not to overwork it or the biscuits can turn out tough.
• Wrap the dough and chill for 30 minutes so it firms up and is much easier to roll.
• Preheat the oven to 180ºc/160ºfan and line 2-3 large trays with paper.
• Lightly flour the work surface
• Roll the dough out into a large rectangle, until it’s about 1/2cm thick all over. Keeping it even is what helps every biscuit bake at the same rate.
• Cut the dough into biscuits, and gently press a patterned cutter to mark the tops if you’re using one.
• Carefully place each biscuit onto the lined trays, spaced apart slightly. 
• Bake for 10-11 minutes. They’ll still feel a little soft in the middle but will firm up as they cool, so don’t be tempted to over bake them.
• Remove from the oven and leave to cool fully on the tray before filling.

Filling

• Add the room temperature butter to a large bowl and beat until soft and pale.
• Add the icing sugar and cocoa powder and beat again until smooth.
• Add the vanilla, and 1 tbsp of whole milk at a time until you reach a smooth and pipeable consistency. 
• On half of the biscuits, pipe a generous amount of buttercream, then sandwich with a second biscuit and press gently so the filling just reaches the edges. Leave to set for a few minutes before serving.