Peel the cucumbers. Dice half of one cucumber in 1/4-inch (0.5cm) cubes and set aside for garnishing. Chop the rest of the cucumbers in big chunks and transfer them to a blender. Add the chopped mint leaves, salt, and pepper. Puree for 10 or 15 seconds at high speed. Add the yogurt to the pureed mixture and pulse until combined – do not overmix. Portion out, garnish with the diced cucumber, cover and chill for 2 hours, and up to overnight.