/**
  * @brief Receive an amount of data in interrupt mode.
  * @param huart UART handle.
  * @param pData pointer to data buffer.
  * @param Size amount of data to be received.
  * @retval HAL status
  */
HAL_StatusTypeDef HAL_UART_Receive_IT(UART_HandleTypeDef *huart, uint8_t *pData, uint16_t Size)

// Start
HAL_UART_Receive_IT(&my_uart_handle, my_buf, 1);

...

// Implement the Rx callback
void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart) {
  // Add the new byte to a circular buffer to be processed by the protocol

  // Register another callback the next time we receive a byte
  HAL_UART_Receive_IT(&my_uart_handle, my_buf, 1);
}