Preheat the oven to 180˚C. Line a 20cm cake tin with parchment paper.
In a bowl, beat the butter and muscovado sugar together until smooth. Add the orange zest, vanilla extract.
Stir in the flour and combine to a smooth paste. Gently push the dough in the cake tin until the paste is 2 cm thick.
Score where you will want to cut the biscuit once baked.
Sprinkle with caster sugar and extra thyme if desired and chill in the fridge for 10 minutes if desired.
Bake in the oven for 15-20 minutes, or until pale golden-brown.
Remove from the oven and set aside to cool on a wire rack.