Fist make the spiced butter as this needs to chill. Put all the ingredients into a small mixing bowl and season well before beating together. Lay out a sheet of wrap and spoon the butter into the centre. Form into a log shape, twisting the ends to seal the butter inside. Refrigerate until needed.
Preheat the oven to 220 C, 200 Fan, Gas 7. Line a roasting tin with baking paper and put in the potatoes with their cut sides facing up. Drizzle generously with olive oil and season well with salt and pepper. Roast for 35-40 minutes until cooked through and nicely browned.
Remove the butter from the fridge and cut into small cubes. Scatter over the potatoes and roast for 5 minutes more until the butter has melted. Remove from the oven and toss the potatoes to coat them well. Serve immediately.