The supererogatory prayer with a single rak'ah is Salatul

The supererogatory prayer with a single rak'ah is Salatul Witr.
Salatul Witr is a voluntary prayer that is performed after the Isha prayer and before the Fajr prayer. It consists of an odd number of rak'ahs (units of prayer), with the minimum being one rak'ah.
This prayer is considered supererogatory, meaning it is not obligatory but highly recommended. It is a way for individuals to earn extra rewards and strengthen their connection with Allah.
Salatul Witr is a special prayer that holds great virtue in Islam. It is often performed separately from the regular daily prayers. It is recommended to perform it during the night, after the Isha prayer, but before the Fajr prayer. However, if someone is unable to wake up for it before Fajr, they can still perform it before going to sleep.
The prayer consists of various rak'ahs, with odd numbers being preferred. The final rak'ah, known as the Witr rak'ah, is the one that distinguishes this prayer from others. It is usually performed with an odd number of units, such as one rak'ah, three rak'ahs, or five rak'ahs, but it can also be performed with more.
During Salatul Witr, additional supplications, known as Qunoot, are made in the final rak'ah. These supplications can be made in Arabic or the person's local language, and they can ask for guidance, forgiveness, and blessings.
Performing Salatul Witr is highly recommended, but it is important to remember that it is not obligatory like the five daily prayers. However, it holds immense spiritual benefits and can strengthen one's relationship with Allah.