Tip the couscous into a large bowl. measure 400ml boiling water into a measuring jug and crumble in the stock cube. Add the butter and stir until the stock cube has dissolved and the butter melted. Pour over the couscous, then tightly cover with cling film. Leave for 10 mins until all the stock has been absorbed.
Meanwhile, heat a large griddle or non-stick frying pan over medium heat. Cut the halloumi into ½cm-thick slices. When the pan is really hot (a drop of water will sizzle immediately), place the halloumi slices in the pan. Cook for 2-3 mins until charred and lightly golden. Use a fish slice to turn cheese and repeat on the other side. Cut the slices in half.
Tip the yogurt into a small bowl. Mix in the harissa, a little at a time, until you're happy with the flavour. Tip the chickpeas into a sieve and rinse under the cold tap. Drain any liquid from the peppers, scraping away any seeds and tear into chunks.
Remove the cling film from the couscous and use a fork to fluff up the grains and break up any clumps. Mix in the chickpeas, peppers and rocket. Toss through the warm halloumi slices, then drizzle over the yogurt dressing to serve.