Mala describes a flavor sensation combining the tingling or numbing effect of Sichuan pepper with chili heat. It is prominent in Sichuan and Chongqing cooking, including hot pots, sauces and dry seasonings.
It is a flavor profile rather than one fixed sauce. A mala paste, oil and spice powder may contain different ingredients and salt levels. Black pepper does not reproduce Sichuan pepper's characteristic numbing sensation.
Reference: Further reading.